One basic rule in manipulating equations is to remember that what you do to one side of an equation, do exactly to the other side as well.
So, here we go:
f = \frac{v}{a}
If we want to have f/2, i.e, we have to divide the LHS by 2, right? So, we should also divide the RHS by 2, like this:
\frac{f}{2} = \frac{\frac{v}{a}}{2} = \frac{v}{2a}---------------------------Now, say we want to solve for a, we will try to isolate a to one side of the equation, like this:
f = \frac{v}{a}
Multiply both sides by a to obtain:
\Rightarrow fa = \frac{v}{a} \times a = v
The LHS has fa, and we want it to be a, so we divide both sides by f, and arrive at:
\Rightarrow \frac{fa}{f} = \frac{v}{f}
\Rightarrow a = \frac{v}{f}