A lightbulb has an efficiency of 8.2%. How much light energy (not heat energy) is generated by the bulb every second if the bulb has a power of 21 W?

The efficiency is the useful energy (or power) you get out of a system, divided by the energy (or power) you put in.

If the efficiency is 8.2%, that means you get 8.2% * 21 W = 1.72 W of useful power out.

Energy is just power multiplied by time, so 1.72 W * 1 s = 1.72 J.