If gabbro cuts across both sandstone and schist and schist is the oldest, draw a geological cross section showing relations between the rocks.
Write about the geological history of the region starting with the oldest events.
I'll leave the drawing up to you as it is pretty simple (two layers with another cutting across) - although be sure to account for the metamorphic rind that will occur from the gabbro's contact
As schist ...
answer gives factors to consider when constructing a geologic history based on a simple cross sectional assemblage of rocks