The answer is A. Because the triangle is a right triangle you can use the equation a^2 + B^2 = C^2. This formula is known as the Pythagorean theorem. A and B are interchangeable as the legs of the triangle so it doesn’t matter which number you use first.
(42•42) + (56•56) =
1764 + 3136 = 4900
So the square root of 4900 is 70.
