There is no formula! The glide slope formulas are not the best way of answering the question, so no angle is needed. This is one you have to work out from your knowledge of navigation. You will not be handed everything in navigation on a plate, as a formula, as the information you learn is too broad in application. The mathematics are not difficult for the question, you simply need the speed / time / distance equation, applied twice.
The common factor is TIME. You must descend and cover the ground in the same time, so that is the first thing to calculate.
If you are given descent and space available and asked for the RoD required, then work out the time from the distance to be covered and the groundspeed. Use this time, and the descent needed, to calculate the rate of descent.
If on the other hand you are given descent and RoD and asked for the distance required to descend then work out the time for descent from the amount of descent and RoD. Use the time and groundspeed to work out how far you will travel in this time.
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