Just sub x in with 5.
So 6(5)^3+8(5) = 6 (125) + 40 = 750 + 40 = 790.
So 790

To solve this proportion, cross multiply.
By cross multiplying, you will multiply 2.75 * (2/9) and 9 * x.
0.61 = 9x
Divide both sides by 9.
x = 0.067... which rounds to 0.068, so your answer is:
D) x
0.068
The first two are correct