Answer :

distribute.
6 times 2x= 12x
6 times 8= 48
12x+48+3=15
12x+51=15
subtract 51 from both sides. 
12x=-36
divide by 12 on each side 
-3=x
6(2x-8)+3=15
12x-48+3=15 Distribute the 6
12x-45=15 Add (-48) and (3)
12x=60 Add 45 to both sides
x=5 Divide both sides by 12