(10 + y)² - y² = 3x + 12
(10 + y)·(10 + y) - y² = 3x + 12
10² + 20y + y² - y² = 3x +12
100 + 20y = 3x + 12
100 - 12 = 3x - 20y
3x - 20y = 88
3x - 20 y - 88 = 0 (I suppose it is that one, simplifyied and in order of the letters, or that it you at the previous step)