The Solution.
Representing the problem in a diagram, we have
By formula,

In this case,

Substituting these values in the formula above, we get

Clearing the bracket, we get



Dividing both sides by 2, we get

Therefore, the correct answer is option C.
Posting screenshot of my answer in the linked question in case it gets deleted.
Step-by-step explanation:
180= 90+30+20x 90+30=120 180-120= 60
20x=60 x=3