Problem 3: Diff of Two Squares (100pts)

写一个代表变量 \( a \) 和 \( b \) 的平方差 \( a^2 - b^2 \) 的算术表达式，来替换掉下面diff_square函数中的下划线。

def diff_square(a, b):
    """Compute the difference of square a and square b.

    Expected result:
    >>> diff_square(3, 2)
    5
    >>> diff_square(3, 4)
    -7
    """
    return ______