The power is the quotient of energy per time: P = Q/t.
Energy in turn is Q = m * c * dT
m is the mass of the fluid in the beaker (500g), dT the difference of the temperatures (difference of 20 and 60 degrees) and c the specific heat capacity that is unknown and missing. By finding out c, you are able to calculate the energy. And and you can determine the specific heat capacity by using the equation for the energy and put this one in the quotient for the power. The steps to this aren't so hard.