y=a×5+b×3+cx-5
cx=y-5a-3b+5
-5c=7-5a-3b+5=12-5a-3b
5c=-12+5a+3b
x=5时,y=5a+3b+5c-5
=5c+12+5c-5
=10c+7
=10a+6b-17