They have isolated y for you in both equations so you can plug either one into the other.
y = 3x - 1
2x + 7 = 3x - 1
isolate the x
7 = x - 1
x = 8
Go back to the original and plug 8 into x to get y.
y = 2 (8) + 7
y = 16 + 7
y = 23
This means at (8, 23) y = 2x+7 is equal to y = 3x-1. In other words they share a point at (8, 23). This is the only solution because for any other x and y they'd not be equal.
