How do i show that v=28x^2 - 2x^3
? ?

You have to prove your work. It can be difficult... But I believe in you! The way to do it, is to do the math out. If you have 2 different exponents, then you'll have to create multiple equations to solve, and use them to find what Y or X is.
Example: 
4y + 8x = 112
Lets do y first. In order to do this, you need to get Y alone.
4y = 112 - 8x
Then, we divide.
y = 28 - 2x
Then insert your y value into the equation: 4(28 - 2x) + 8x = 112
Solve that, and you will have your x value! Then plug in your x value to find the y value. After that, insert both to check your work. There you go! That's the answer to finding the x. Then plug in your new x value, and complete the equation! Make sure to show all of your work!