Terry can run 15 miles in the same amount of time it takes mark to run 21 miles. if mark runs 2 mph faster than terry, how fast does mark run?

We have the formulas 15 = v1 x t; 21 = v2 x t,  where v1 is the velocity of Terry, v2 is the velocity of Mark and t is the time;
Then v2 = v1 + 2;
v2 x t = (v1 + 2) x t;
So, 21 = 15 + 2xt;
6 = 2xt;
t = 3h;
v1 = 5mph;
v2 = 7mph;

What are the following algebraic equations. you need to post those if you want the answer