To solve for \( y \) in the given system of equations, we need to first express the equations clearly:
- \( x + 6y = 10 \)
- \( -x + 3y = -15 \)
Let's first add these two equations together to eliminate \( x \):
\[ (x + 6y) + (-x + 3y) = 10 + (-15) \]
This simplifies to:
\[ 0 + 9y = -5 \]
So we have:
\[ 9y = -5 \]
To solve for \( y \), divide both sides by 9:
\[ y = -\frac{5}{9} \]
Thus, the value of \( y \) is:
\[ \boxed{-\frac{5}{9}} \]