A voltage source of internal impedance $${\mathrm R}_\mathrm s\;+\;{\mathrm{jX}}_\mathrm s$$ supplies power to a load of impedance $${\mathrm R}_\mathrm L\;+\;{\mathrm{jX}}_\mathrm L$$ in which only $${\mathrm R}_\mathrm L$$ is variable. Determine the value of $${\mathrm R}_\mathrm L$$ for maximum power transfer from the source to the load. Also, find the numerical value of $${\mathrm R}_\mathrm L$$ if the source impedance is 3.0 Ω (purely resistive) and $${\mathrm X}_\mathrm L$$ is 4.0 Ω.