diff --git a/numpy_questions.py b/numpy_questions.py
index 07a10c1..0ba5246 100644
--- a/numpy_questions.py
+++ b/numpy_questions.py
@@ -40,7 +40,9 @@ def max_index(X):
     i = 0
     j = 0
 
-    # TODO
+    if not isinstance(X, np.ndarray) or X.ndim != 2:
+        raise ValueError("X must be a 2D numpy array")
+    i, j = np.unravel_index(np.argmax(X), X.shape)
 
     return i, j
 
@@ -62,6 +64,9 @@ def wallis_product(n_terms):
     pi : float
         The approximation of order `n_terms` of pi using the Wallis product.
     """
-    # XXX : The n_terms is an int that corresponds to the number of
-    # terms in the product. For example 10000.
-    return 0.
+    if n_terms == 0:
+        return 2.0
+    n = np.arange(1, n_terms + 1)
+
+    res = (4 * n**2) / (4 * n**2 - 1)
+    return 2 * np.prod(res)