Question

[5-12:(6+a:3-2)]•7+3=24

Answer 1

[5-12:(6+a:3-2)]x7=24-3=21
[5-12:(6+a:3-2)]=21:7=3
12:(6+a:3-2)=5-3=2
6+a:3-2=12:2=6
6+a:3=6+2=8
a:3=8-6=2
a=2x3
a=6

Answer 2

$[5-12:(6+a:3-2)]*7+3=24$
$[5-12:(6+a:3-2)]*7=24-3$
$[5-12:(6+a:3-2)]*7=21$
$[5-12:(6+a:3-2)]=21:7$
$[5-12:(6+a:3-2)]=3$
$12:(6+a:3-2)=5-3$
$12:(6+a:3-2)=2$
$6+a:3-2=12:2$
$6+a:3-2=6$
$6+a:3=6+2$
$6+a:3=8$
$a:3=8-6$
$a:3=2$
$a=2*3$
$\boxed{a=6}$