A resistor develops 300J of thermal energy in 15s , when a current...

Energy produced is given by $H = {i}^{2}Rt$

When $i = 2A, t = 15s$, energy is $H = 300 = {2}^{2} \times R \times 15$

Resistance is $R = 5Ω$

When $i = 3A, t = 10s$.

Energy is ${H}^{'} = {3}^{2} \times 5 \times 10$$= 450J$