Answer:
mB=12-x
Step-by-step explanation:
mA+mC+mB=180
9x-12+5x+24+mB=180
9x+5x-12+24+mB=180
14x+12+mB=180
14x+mB=180-12
14x+mB=168
x+mB=168÷14
x+mB=12
