So as far as my knowledge goes, your answer for (a) is right - it is a cylinder.
(b) is a triangular pyramid - this is because the base is a triangle ( aka the one in the middle) and there are three triangles surrounding it and when they come together, they form a triangular pyramid.
hope it helped, if it did, please make me the brainliest answer - I need to move on to the next level :)
