put all the angles = 180 (for a triangle's sum is 180 degrees)
x - 4 + 100 + 3x = 180
Add numbers (& common variables) together
3x + x + 100 - 4 = 180
4x + 96 = 180
isolate the x, subtract 96 from both sides
4x + 96 (-96) = 180 (-96)
4x = 180 - 96
4x = 84
isolate the x, divide 4 from both sides
4x = 84
4x/4 = 84/4
x = 84/4
x = 21°
hope this helps