We have:
x - 3y = 4
Pass -3y to the second member of equation
x = 4 + 3y
Now replace on the other equation tge value of x
2x + y = 7
2.(4 + 3y) + y = 7
Do the diatributive
2 ×4 + 2×3y + y = 7
8 +6y+y = 7
8 + 7y = 7
7y = 7 - 8
7y = - 1
y = -1/7
Now replace y = -1/7 in any equation
x = 4 + 3y
x = 4 + 3 . -1/7
x = 4 -3/7
x = (4×7 -3)/7
x = 25/7