spot the errors and explain.

Answer:
student c
9x³ error
-6x³+15x²+14x-11 answer
student d
14x²-27 error
-6x³+15x²+14x-11 answer
student e
13x²+14x+15 error
9x³-17x²-23 answer
student f
x-15 error
9x³-17x²-23 answer