You are correct, y would equal -1
But let me show you why
-5y+8=-3y+10
----------------------------------------
Add 3y to each side
-5y+8+3y=-3y+10+3y
-2y+8=10
------------------------------------------------
Subtract 8 on each side
-2y+8-8=10-8
-2y=2
--------------------------------------------------------
Now divide each side by -2
-2y ÷ -2 = 2 ÷ -2
y = -1
==================================================================
Hope I helped, Have a great day! :D