One-third a number (x) added to one-half the number equals nine less than the number
the expression:

The answer to your question is:

1/3x + 1/2x = x - 9

Answer: 8x^3-7x^2-6x+5
a cubic expression is one that the highest exponent or power is 3 or cubed. the first two the highest exponent is only x or x^2 so they aren’t cubic. the first one is a linear expression and the second one is a quadratic expression. the third choice is correct as its highest power is x^3 so this is a cubic expression. and the last choice isn’t correct as it is a quartic expression or a polynomial of degree 4. i hope this explanation and any questions feel free to ask.