Substitute (3x - 1) for y in the first equation
3x - 1 = 2x + 8
x - 1 = 8 - subtract 2x from each side
x = 9 - add 1 to each side
Substitute 9 in for x in one of the equations
y = 2x + 8
y = 2(9) + 8
y = 18 + 8
y = 26
Check
y = 3x - 1
26 = 3(9) -1
26 = 27 - 1
26 = 26