I set it in a big problem. Since you know that all the angles of a triable add up to 180,
m<a + m<b + m<c = 180, plug in equations/values
(36) + (3x+12) + (3x+18) = 180, subtract 36
3x+12 + 3x+18 = 144, combine like terms,
6x+30 = 144, subtract 30,
6x=114, divide by 6,
x=19. Plug in X to the equations for m<b and m<c