Answer:
make Y the subject in eqn........ 1
y + 7 = 2x
y = 2x - 7...........eqn 3
put y = 2x - 7 into eqn 2
x² - xy + 3y² = 15
x² - x(2x - 7) + 3(2x - 7)(2x - 7) = 15
x² - 2x² + 7x + 3(4x² + 14x -14x + 21) = 15
x² - 2x² + 7x + 12x² + 42x - 42x + 63 = 15
x² - 2x² + 7x + 12x² + 63 = 15
x² - 2x² + 12x² + 7x + 63 = 15
11x² + 7x + 63 - 15 = 0
11x² + 7x + 48 = 0
11x² + 7x = - 48
11x²/11 + 7x/11 = - 48/11
x² + 7x