Step-by-step explanation:
you don't understand how a function works ?
because basically, once you understand it, this is totally trivial.
a function f(x) works in that sense that whatever value you have for x, you put that value instead of the variable name into the expression (at every position of the variable name). and then you simply calculate. that's it.
so, for f(2) we need to put that argument (2) into the definition instead of "x".
f(2) = 12.9 - 2×2 = 12.9 - 4 = 8.9
