x+2=5y / 3y=12 / 2y=x+2z are essentially your equations
from the 2nd one, 3y=12, we know y=4, or in your case, circle = 4
we can then plug 4 into x+2=5y to get x+2=20, or x=18, which means diamond = 18
then, plug 4 and 18 into 2y=x+2z for 2(4)=18+2z / 8=18+2z / -10=2z / -5=z, so triangle = -5
