An amplifier circuit employs a transistor in common-emitter mode,...

The power gain of a common-emitter amplifier is given by the product of the square of the current gain and the resistance ratio.

Power gain $A_p = \beta^2 \times \dfrac{R_L}{R_{in}}$

Given values are:
Current gain $\beta = 50$
Output resistance $R_L = 2 \text{ k}\Omega$
Input resistance $R_{in} = 0.50 \text{ k}\Omega$

Substituting the values into the formula:

$A_p = (50)^2 \times \dfrac{2}{0.50}$

$A_p = 2500 \times 4 = 10000 = 10^4$

The change in input current is extra information and is not required to find the power gain.