m = (y - y1)/(x - x1)
Multiply both sides by (x - x1)
(x - x1)*m = (x - x1) * ((y - y1)/(x - x1)
(x - x1)*m = (y - y1)
Remove parenthesis using distributive property:
mx - mx1 = y - y1
Subtract y from both sides:
mx - mx1 - y = y - y1 - y
mx - mx1 - y = -y1
Multiply both sides by (-1)
(-1)(mx - mx1 - y) = (-1)-y1
y + mx1 - mx = y1
y1 = y + mx1 - mx
