solve

I assume that you meant -6 + f = 2f - 8

$-6+f = 2f-8\\ f-6=2f-8\\ f+2=2f\\ 2 =f\\ f=2$

If you meant:

Does     -6 + f = 2     if     f = 8         ?

-6 + f   $\stackrel{?}{=}$   2

Replace  " f "  with  " 8 "  .

-6 + 8   $\stackrel{?}{=}$   2

Simplify left side of equation.

2   $\stackrel{?}{=}$   2

2   =   2        true

Yes,     -6 + f = 2     if     f = 8  .