We know that the interior angles of a triangle add up to 180 degrees. We already have one angle, the bottom one, so let's just get that out of the picture by subtracting it from 180. The other two angles add up to be 80 degrees. What we can do is combine them together and get 4x-4=80. We have 4x because of x+3x. It equals 80 because that's what the two angles together add up to be. What we want to do now is get x by itself. Subtract 4 from both sides and get 4x=76. Now, we divide by 4, to undo multiplication. We are left with x=19, and now we have our answer.
