This is a system of equation type of problem. There are two ways of solving problems such as these: substitution and elimination.
In this problem, we are already given x.
x = y + 3
So, all we have to do is substitute y + 3 for x in the other equation.
6x - 5y = 15
Substitute
6(y + 3) - 5y = 15
Distribute
6y + 18 - 5y = 15
Combine like terms
y + 18 = 15
Subtract 18 from both sides.
y = -3
Now that we know y = -3, we can solve for the numerical value of x.
Just substitute -3 for y in either equation.
1) 6x - 5(-3) = 15
6x + 15 = 15
6x = 0
x = 0
2) x = y + 3
x = -3 + 3
x = 0
The solution for this system of equation is (0, -3).