I don't have my calculator handy just now. But just looking at that problem, why don't you try +20 and -45 and see if they do the job.
Answer:

Step-by-step explanation:
It's like solving any equation, except we have letters instead of numbers .
k = 3tm + rs {subtract rs from both sides}
k - rs = 3tm {divide both sides by (3m)}
{swap sides}
m≠0
{m must be ≠0 because you cannot divide by 0. There is no t for m=0}