Hello Raul: see if this helps:
for case 1, power P =V(volts)* I(amperes), P = 12*8=96W
for case 2, you have to determine whether you can use any of the three Ohms Law formulas for power: P=VI, P=V^2/R, P=I^2*R. Because you are only given one measurement, the voltage V, and I is not defined, and R is not defined, you cannot find the total power with Ohms Law. The answer is allready given in implied form:
total Watts = P = (30W + 30W + 30W + 30W) =120W
for case 3 you have two measurements and so can use
P = VI for each bulb: P=24*3=72W per bulb, since you have two bulbs the total P = 2(24*3)= 144W.
The most power is used in case 3.