Answer:
mB=12-x
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=168รท14
x+mB=12
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