How do I start this problem?

Since you listed this as basic chemistry I'm going to assume that you're using the simplified equation

ΔT_{f} = -K_{f}*C_{m}*i

You've got values for

ΔT_{f} = -2.5°C

K_{f} = 1.853 (K*kg)/mol

i = 1 because it is a non-electrolyte, there's no ionization

The only other trick would be to remember that molal concentration (C_{m}) is moles of solute per kg of solvent, and then that moles of solute is equal to

**5g solute*(1 mol/x g solute)**.

so you end up with

**2.5 = 1.853*[(5/x)/(0.025 kg)]**

Just solve for x.