This is a bit hard and I’ve tried multiple times and still don’t get it

Given the following function:
[tex]f(x)=x-7[/tex]to find f(10), we have to make x = 10 and substitute the value on the function to get the following:
[tex]\begin{gathered} f(10)=10-7=3 \\ \Rightarrow f(10)=3 \end{gathered}[/tex]therefore, f(10)=3