Answer:
Can't draw a flow chart here, but here are the steps you need:
1) Prompt for side length of cube
2) If the entry is non-numeric, give error and go to step 1
3) If the entry is less than zero, give error and go to step 1
4) output "Volume is " + (entry * entry * entry)
5) output "Surface area is " + (6 * entry * entry)
6) Prompt for calculating another cube
7) If the reply is "y", go to step 1
8) end
