5g ? 3=12,

Dear Student, 
your question is unclear and appears to be incomplete. Recheck your question and please be little specific about the ? sign to which you are referring here so that we can provide you with some meaningful help. 
But assuming as follows
if it is 5g + 3 = 12, then 
5g = 12 - 3
5g = 9
g = $\frac{9}{5}$
OR 
if it is 5g - 3 = 12, then
5g = 12 + 3
5g = 15
$g=\frac{15}{5}$
g = 3
Look forward to hearing from you again!
Regards!