Okay, take for example:
1+2=3 is the same as 1=3-2
See how we changed the place of the 2. We can do that as long we change + to - and vica versa.
Let's see your example:
6+y=-8
We move the 6 on the other side of the expression with the opposite sign (it is a rule) and we get:
y=-8-6
y=-14
The answer to your question is
86+y=99
y=99-86
y=13
