Will mark brainliest !!!!!!!!!!!!!! Answer correctly please !!!!!!!!!!!!!!!!!!!!!

Answer:
x = 39
Step-by-step explanation:
The square on the angle means that angle is 90 degrees, also known as a right angle. A triangle is 180 degrees so all the sides added together is 180:
51 + 90 + x = 180
141 + x = 180
x = 39