Me don’t know how to do this and i have psat in a week.

Use the quadratic formula to solve for x (see attached image; we plug in a = 2, b = 7 and c = -15). Doing so leads to 
x = 3/2
x = -5

Since 3/2 = 1.5 is larger than -5, this means r = 3/2 = 1.5 and s = -5

Subtract r and s:
r - s = (3/2) - (-5)
r - s = (3/2) + (5)
r - s = (3/2) + (5/1)
r - s = (3/2) + (10/2)
r - s = (3+10)/2
r - s = 13/2

Therefore the final answer is B) 13/2

The operation is suck a c@ck? !