seven subtracted from the product of four and a number is a most 26 use the variable C for the unknown number

We’ll break this down:

“7 subtracted from” means you’ll take 7 away from something.

That something is “the product of 4 and a number.”
Product means multiplication, so you need to multiply 4 and a number, which you’re told to call C.
“The product of 4 and a number” becomes 4•C or just 4C.

Putting those together “seven subtracted from the product of 4 and a number” becomes:
4C-7

“Is at most” also can be stated as “no more than.” This means your 4C-7 cannot be bigger than 26. If it cannot be bigger than 26, then it needs to be less than or equal to 26.

4C-7 ≤ 26.

The third box is a function