in 1., do I need to use √(WR) ? Yes
like it'd be square of (0,320W times 200 Ω) = 64 volts? Your numbers are wrong and I think you mean square root.
in the 2nd one. (I'll sleep on this one and try it in the morning) Good because I have no idea what the question means!
Finally the 3rd one, a) is it simply V=RI (so 6 volts because 15Ω x 0.4 A) ? Yes
but here's the one bothering me: energy dissipated in the resistor in one minute, I need an explanation on this one if someone has some time to teach this here !
Energy is measured in Joules.
A Joule is a watt.second, so 5 watts for 3 seconds is 15 Joules.
Does that help?