How do you evaluate f(x)=10-2x for f(1)?

$f \left(1\right) = 8$
$\text{substitute x = 1 into } f \left(x\right)$
$\Rightarrow f \left(\textcolor{red}{1}\right) = 10 - \left(2 \times \textcolor{red}{1}\right) = 10 - 2 = 8$