Find the solution of this system of equations x-7y=-20 -3x+9y=36

Solve the following system using elimination:
{x - 7 y = -20 | (equation 1)
{9 y - 3 x = 36 | (equation 2)

Swap equation 1 with equation 2:
{-(3 x) + 9 y = 36 | (equation 1)
{x - 7 y = -20 | (equation 2)

Add 1/3 × (equation 1) to equation 2:
{-(3 x) + 9 y = 36 | (equation 1)
{0 x - 4 y = -8 | (equation 2)

Divide equation 1 by 3:
{-x + 3 y = 12 | (equation 1)
{0 x - 4 y = -8 | (equation 2)

Divide equation 2 by -4:
{-x + 3 y = 12 | (equation 1)
{0 x+y = 2 | (equation 2)

Subtract 3 × (equation 2) from equation 1:
{-x+0 y = 6 | (equation 1)
{0 x+y = 2 | (equation 2)

Multiply equation 1 by -1:
{x+0 y = -6 | (equation 1)
{0 x+y = 2 | (equation 2)
Collect results:
   {x = -6
                 {y = 2

(x) - (y) = (1 + 4i)

(3 + 6i) - (2 + 2i) = (1 + 4i)