a) 

$\frac{1}{2}x=12$

$\frac{1}{2}\cdot 6=12$

$3\ne 12$

b) 

$36=\frac{1}{3}\cdot 108$

$36=36$

c) 

$9\cdot 12+3=7\cdot 12+5$

$108+3=84+5$

$111\ne 89$

d)

$13\cdot 3-8=9\cdot 3+1$

$39-8=27+1$

$31\ne 28$