$$\frac{2.5\times2.5+2\times3.75+1.5\times1.5}{2.5+1.5}=f4arll$$
$a=\frac{4}{f_{4}rl^{2}}$
$l\neq 0\text{ and }r\neq 0\text{ and }f_{4}\neq 0$
$f_{4}=\frac{4}{arl^{2}}$
$l\neq 0\text{ and }r\neq 0\text{ and }a\neq 0$