you have two equations here, where x is the "max" allowed and y is the "remainder"

x+y = 10,000

.07x+.0825y = 725

solve top for x and then plug into the second:

x = 10,000-y

.07(10,000-y)+.0825y = 725 ==> .0125y+700=725 ==> y=2,000

so then x+2000=10,000 ==> x=8,000

your equations here seem right to me. now all you would do is solve for x in the equation 630 = (6 + .5x)(96 - 4x). i got x = 3 or x = 9. so he could charge either $7.50 or $10.50 to make $630 per week. the question doesnt say if the price or the tapes sold should be minimized or maximized, so either answer is right.