How do you solve 2mn-2mt plus 2sn-2st equals 0?

2mn - 2mt + 2sn - 2st = 0

Divide both sides of the equation by 2:

mn - mt + sn - st = 0

m (n - t) +s (n - t) = 0

(m + s) (n - t) = 0

That's as far as you can go with the information given.