The answer to this answer choice is A. (45x12)+75=c
Example: 
We can see that there is more than one number with the variable x, therefore, we say they're ''like terms'' and because of that they can be summed. We do this with all of the other numbers with similar variables. If no numbers with similar variables are left, like 4a, you don't do anything but write them as they are. You can also see that 8 and 9 can also be summed because neither of them has a variable, therefore they're similar.
In this step, you just do the operation with the numbers and keep the same variable.


since there are not more numbers similar in variables, this operation is done.

In f(x) = 3x + 2, the x is being represented in the function. So, if we replace the x with the g(x) functions, we get:
f(g(x)) = 3(2x - 4) + 2
f(g(x)) = 6x - 12 + 2
f(g(x)) = 6x - 10
I think you are missing part of the problem. If you had another equation or a value for x you could solve it.
The formula of the volume of a cylinder:

We have:

and

substitute:
Answer: The area of a base of the cylinder is equal 45 dm².