How do you solve y+mx1=mx+y1 for m

y+mx1=mx+y1
Subtract y1:
y-y1+mx1=mx
Subtract mx1:
y-y1 = mx - mx1
Factor out m from the right:
y-y1 = m (x - x1)
Divide by (x - x1):
m = (y-y1) / (x - x1)