Find the value of x. I need help I don’t understand how to do this.

Answer:
x = 12.
Step-by-step explanation:
This is a right angled triangle so you use the Pythagoras Theorem:
15^2 = 9^2 + x^2
x^2 = 15^2 - 9^2
x^2 = 225 - 81
x^2 = 144
x = √144
x = 12.