The formula y-y1=m(x-x1) is known as the point-slope form to graph a line. Solve this formula for x.
x = (y-y1)/m + x1
y-y1=m(x-x1)
y - y1 = mx - mx1
y - y1 + mx1 = mx
x = (y - y1 + mx1)/m
y - y1 = mx - mx1
y - y1 + mx1 = mx
x = (y - y1 + mx1)/m