To solve by substitution, you need to fill in the value for either x or y. Since y = x + 5, you already have the y value. It's x + 5. Now you need to find x before you can go any further. So let's take 5x + y = 3 and fill x + 5 in for y.
5x + y = 5
5x + x + 5 = 3
Now solve for x.
5x + x + 5 = 3
6x + 5 = 3
6x = 3 - 5
6x = -2
x = -2/6 or -0.33
Now solve x + 5 for y.
y = x + 5
y = -0.33 + 5
y = 4.67
So x = -0.33 and y = 4.67
