x + 4 = 15
- 4 -4
x = 11
You would subtract 4 because you need to isolate the x to get the value of x. You would subtract it to both sides because whatever you do on one side, you have to do it to the other side.
Given:
A figure.
To find:
The value of x.
Solution:
Draw two parallel lines as shown in the below figure.
If a transversal line intersect two parallel lines, then alternate interior angles are same.
(Alternate interior angle)
(Alternate interior angle) ...(i)
The value of angle d is



Using (i), we get

If a transversal line intersect two parallel lines, then same sides interior angles are supplementary angles.



Now,



Therefore, the value of x is 70.