Answer:
x = √30
Explanation:
Calculate the length of the larger triangle's missing side (or the hypotenuse of the smaller triangle). Use the Pythagorean Theorem because both triangles are right triangles. Let y represent the larger triangle's missing side length.
5² + y² = 8²
25 + y² = 64
y² = 39
y = √39
Now use this value to calculate x.
x² + 3² = (√39)²
x² + 9 = 39
x² = 30
x = √30