Please help explain on how to do it as I am having trouble solving it.

Answer:
the value if x is 15
Step-by-step explanation:
Use Pythagoras theorem
x² = (x - 3)² + 9²
x² = x² -3x -3x + 9 + 81
x² = x² -6x + 90
x² - x² + 6x = 90
6x = 90
divide through by 6
x = 90/6
x = 15