The three inside angles of a triangle need to equal 180 degrees.
One angle is given as 100, so the other two need to equal 180-100 = 80 degrees.
So now you have:
x-4 + 3x = 80
Combine like terms"
4x - 4 = 80
Add 4 to each side:
4x = 84
Divide both sides by 4:
x = 84/4
x = 21
