diff --git a/numpy_questions.py b/numpy_questions.py
index 07a10c1..81e700b 100644
--- a/numpy_questions.py
+++ b/numpy_questions.py
@@ -40,8 +40,12 @@ def max_index(X):
     i = 0
     j = 0
 
-    # TODO
+    if not isinstance(X, np.ndarray):
+        raise ValueError("Input must be a numpy array")
+    if X.ndim != 2:
+        raise ValueError("Input array must be 2D")
 
+    i, j = np.unravel_index(np.argmax(X), X.shape)
     return i, j
 
 
@@ -56,12 +60,12 @@ def wallis_product(n_terms):
     n_terms : int
         Number of steps in the Wallis product. Note that `n_terms=0` will
         consider the product to be `1`.
-
-    Returns
-    -------
-    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, dtype=float)
+    factors = (4 * n * n) / (4 * n * n - 1)
+    approx_pi = 2.0 * np.prod(factors)
+
+    return approx_pi