QUESTION 1: FIND X
The key is to make the resultant force (I call it original force) as hypotenuse. Here, 100N is the original force.
Let's go for next example:
QUESTION 2: FIND X & Y
This time, can you find the original force? yup, it's the weight = mg = 100N. Hence, make this 100N as the hypotenuse:
Let's compare this with students' common mistake:
Can you see why this is wrong? yup, because the resultant force is not the hypotenuse. Try to remember that original force is the hypotenuse next time :-)