y=x-3 x+2y=6

Replace y in the second equation by x - 3 (because the first equation says this is what y is equal to):

x +2(x-3) = 6

Multiply through the bracketed term by 2:

x + 2x - 6 = 6

Collect the x terms

3x - 6 = 6

Add 6 to both sides

3x = 12

Divide both sides by 3

x = 4

Put this back in the first equation

y = 4 - 3

y = 1