13. we are given an equation
x - x - xy
but also given what x and y are equal to
x = -8
y= 9
we can plug in x and y for the values we are given and solve
-8 - (-8) - (-8 ·9)
- 8 -(-8) -(-72)
- 8 + 8 + 72
72
15. (r+r)(r-q) where q= 6 and r= -2
(-2 + -2) ( -2 - 6)
(-4)(-8)
32
17. y-x²+y where x = -7 and y = -8
-8 - (-7)² + (-7)
-8 - 49 + -7
-57 + -7
-64
19. a-(b²-b) where a = 10 and b = 7
10-(7²-7)
10-(49-7)
10-42
-32
