Answer:
x = √5
Explanation:
The Pythagorean theorem tells you the square of the diagonal is the sum of the squares of the two sides:
(√7)² = (√2)² + x²
7 = 2 + x² . . . . . . simplify
5 = x² . . . . . . . . . subtract 2
√5 = x . . . . . . . . . take the square root