After substituting $x=10$, simplify each parenthesis separately: one involves $10 + 0.2\cdot 10$ and the other involves $60 - 2\cdot 10$.

Once both parentheses are simplified to single numbers, multiply those two numbers together to get $g(10)$. Be careful with the decimal $0.2$.

In Desmos, type g(x) = (x + 0.2x)(60 - 2x) to define the function $g$.

In a new line, type g(10) and look at the numerical value Desmos gives. That number is the value of $g(10)$ and matches one of the answer choices.

The notation $g(10)$ means: take the formula for $g(x)$ and replace every $x$ with $10$.

So start from

and write an expression for $g(10)$ by substituting $x=10$ everywhere.

Replace each $x$ with $10$:

Now simplify inside each set of parentheses.

First factor:

So the first parenthesis becomes $12$.

Now simplify the second factor:

So now you have

The last step is to multiply these two numbers.

Compute the product:

So $g(10) = 480$, which corresponds to answer choice C.