STEP - BY - STEP EXPLANATION
What to find?
Determine the method to use when solving the given triangle.
Given:
Condition for using inverse trigonometric
We can go for this choice if what we are looking for is an angle.
So since we are looking for the side x of the triangle, we can go for this option.
Condition for using trigonometry
We can use this option, if at least one of the angle is given together with angle 90.
Condition for using Pythagorean theorem
We opt for this option if it is a right triangle and two of the sides are given.
Clearly, it is a right triangle and two sides are given.
ANSWER
Pythagorean Theorem