5. f(x) = 4 . 7^x
for x = -1
f(-1) = 4 . (7)^-1 = 4/7
for x = 2
f(2) = 4 . (7)^2 = 4 . 49 = 196
17. x^2 - 8x = 9
x^2 -8x + 16 = 9 +16
(x - 4)^2 = 25
(x - 4) = +/-5
x = 5 + 4 = 9 and x = -5 + 4 = -1
18. 3x^2 -5x + 4 = 0
Discriminant √(b^2 - 4ac) = √[(-5)^2 - 4(3)(4)] = √[25 - 48] = √(-23)
Given that √(-23) is an imaginary number, the equation does not have real solutions.