Bismuth's specific heat is 121 J/kgK, the lowest of any non-radioactive metal. What is the mass of a bismuth sample if 25 J raises its temperature 5.0 K?"

Use the formula:

Q = mc\Delta \theta

where Q is the Quantity of heat
m is the mass
c is the specific heat capacity
Delta theta the change in temperature.

You have Q, c, and delta theta. Solve for m, the mass.