Please help! It's grade 12 college math. #7a and b. Show all work including formulas, concluding statements and include proper units. Thanks soo much!

7. a) Determine the volume of the concrete needed to build the platform shown in the diagram. [3 marks] 15 cm 20 um b) What is the surface area of the exposed sides of the platform (top and sides only - not bottom). (Don't forget the front and back.) [3 marks]