There is a typo in the question, but here is the general idea:
So this is the equation you wrote: 6x+5y=-15
you have to re-organize everything so only the Y is on the left side of the equals sign.
so the 5y is already on the left, so all we have to do is move the 6x to the other side. to do that, you
subtract 6x from the left so it moves to the right
6x
-6x+5y=-15-
6xby subtracting the 6x on the left, both of the 6x just disappear and it moves to the other side of the "="
so now your equation looks like this:
5y=-6x-15
all you have to do now is divide by five (so it just y instead of 5y)
then you simplify and your final answer is: