7x - 3y = 1 => 3y = 7x - 1 => y = (7x - 1)/3
2x + 5y = 12
2x + 5 * (7x - 1)/3 = 12
2x + (35x - 5)/3 = 12
6x + 35x - 5 = 36
41x - 5 = 36
41x = 41
x = 41 : 41
x = 1
y = (7 - 1)/3
y = 6/3
y = 2
