Answer
$4.6cm$
Work Step by Step
As $d_i=-md_{\circ}$
$\implies d_i=-(2.5)(2.76)=-6.9cm$
Now we can find the focal length as follows:
$f=(\frac{1}{d_{\circ}}+\frac{1}{d_i})^{-1}$
We plug in the known values to obtain:
$f=(\frac{1}{2.76}+\frac{1}{-6.9})^{-1}=4.6cm$