A level quadratics concerning inequalities HELP!?!?!?

The quadratic equation (k+1)x^2+12x+(k-4)=0 has real roots. a) Show that k^2-3k-40≤0. [3 marks] b) Hence find the possible values of k. [4 marks] I appreciate the fact that that's a lot of help I'm asking for but I honestly have no clue!!!! :( Therefore, any help (with explanations) would be hugely appreciated!!!! Thanks in advance!!!!! :D