x+y=7
x=y+3
Replace x with y + 3 in first equation.
y + 3 + y = 7
2 y + 3 = 7
Subtract 3 to both sides
2 y = 4
y = 4 / 2 = 2
x = y + 3 = 2 + 3 = 5
x=y+3
y + 3 + y = 7
2 y + 3 = 7
Subtract 3 to both sides
2 y = 4
y = 4 / 2 = 2
x = y + 3 = 2 + 3 = 5