Look at these photographs. Where can you buy these products?
I´m going to the greengrocer´s to buy a some bananas.
This answer is fine, as far as it goes. However, it doesn't complete the task! You were asked about all three photographs! Perhaps you would like to complete the task, and I will happily make any further corrections!! :-)
I´m going to greengrocers to buy bananas.
(note: you use the helpful a if it was only one banana).