"r varies jointly as p and q" means r = pq, and inversely with t means r = pq/t, r = -1, p=3, q= -7, t=8

-1 = (3)(-7)/8 * x

-1 = -21/8 x

8/21 * -1 = -21/8 * 8/21 x

-x = -8/21

x = 8/21

So equation would be r = 8pq/21t

"varies jointly" means two terms are directly related so r = pq, if r is "varied inversely" with t, r = 1/t (inverse means take the term into a denominator of a fraction)

All I did was to put the numbers in and solved for "x" so that the numbers equal on both sides. Solved x by multiplying both sides by 8/21.

Let me know if you have any other questions like this.