$a)3\left(x-1\right)=x+7$

$3\left(5-1\right)=5+7$

$3\cdot 4=12$

 

$12=12$

$b)2\left(x-1\right)+4=3x-4$

$2\left(5-1\right)+4=3\cdot 5-4$

$2\left(5-1\right)+4=3\cdot 5-4$

$2\cdot 4+4=15-4$

$12\ne 11$