Hello again Charlesbabegirl! :)
First of all, let's say that the sum of all internal angles of a triangle is always 180°
This means that if you add up angles A, B and C you get 180 but we should find first the value of x by setting up a proportion of this type:
A + B + C = 180
12x + 12 + 15 + 3x + 18 = 180
Move all the known terms (those that don't have x) to the right with changed sign.
12x + 3x = 180 - 12 - 15 - 18
15x = 135
Divide both terms by 15
x = 9
Then plug the value of x...
m∠A = 12(9) + 12 = 108+12 = 120°
m∠C = 3(9) + 18 = 27 + 18 = 45°
Hope I helped and let me know if I was right!
