diff --git a/README.md b/README.md
index 4228db5..bdc43a8 100644
--- a/README.md
+++ b/README.md
@@ -45,6 +45,18 @@ intersection_result = intersect_triangles(t1, t2)
 plot(intersection_result)
 ```
 
+```ruby
+p1 = Point.new(2.2, 3.3)
+p2 = Point.new(5.5, 6.6)
+ls = LineSegment.new(p1,p2)
+# find a length of a line segment based on two points
+lenght = ls.length
+# return straight line object
+StraightLine sl = ls.to_straight_line
+
+
+```
+
 ![triangles_example](docs/images/triangles_example.png)
 
 ![intersect_triangles_example](docs/images/intersect_triangles_example.png)
diff --git a/lib/primitives/elementary/line_segment.rb b/lib/primitives/elementary/line_segment.rb
index 00f4924..90e5d02 100644
--- a/lib/primitives/elementary/line_segment.rb
+++ b/lib/primitives/elementary/line_segment.rb
@@ -5,13 +5,31 @@ class LineSegment
       attr_reader :ends_at
 
       def initialize(begins_at, ends_at)
-        # Your code goes here...
+        check_is_a_point(begins_at)
+        check_is_a_point(ends_at)
+        @begins_at = begins_at
+        @ends_at = ends_at
       end
 
+      # Count a length of a line segment based on two points
       def length
-        # Your code goes here...
-        # REQUIREMENT: use lazy evaluation
+        @length ||= begins_at.distance ends_at
       end
+
+      # Return straight line object based points coordinates
+      def to_straight_line
+        a = ends_at.y - begins_at.y
+        b = begins_at.x - ends_at.x
+        c = -begins_at.x * ends_at.y + begins_at.y * ends_at.x
+        StraightLine.new(a, b, c)
+      end
+
+      private def check_is_a_point(x)
+        unless x.is_a? Point
+          fail TypeError.new "Invalid type of argument"
+        end
+      end
+
     end
   end
 end
diff --git a/test/elementary_test.rb b/test/elementary_test.rb
index d41847c..f3b1d64 100644
--- a/test/elementary_test.rb
+++ b/test/elementary_test.rb
@@ -1,4 +1,4 @@
-require "test_helper"
+require "./test/test_helper"
 
 class ElementaryTest < Minitest::Test
   include Primitives
@@ -7,6 +7,7 @@ class ElementaryTest < Minitest::Test
 
   def setup
     @point = Elementary::Point.new(2.3, 5.1)
+    @line_segment = Elementary::LineSegment.new(Elementary::Point.new(2.2, 3.3),Elementary::Point.new(5.5, 6.6))
   end
 
   def test_point_attributes_set_correctly_on_instance_create
@@ -42,4 +43,14 @@ def test_move_method_doesnt_affect_point_instance_on_which_it_was_applied
     assert_in_delta 2.3, original.x, @@delta
     assert_in_delta 5.1, original.y, @@delta
   end
+
+  def test_line_length
+    length = @line_segment.length
+    assert_in_delta 4.66690, length, @@delta
+  end
+
+  def test_to_straight_line
+    # test will be added when straight line class is approved
+  end
+
 end