Hi, I'm trying to figure out the explanation for these answers, I tried to work these out and got different answers from what was posted. However, I can't quite figure out what I did wrong. I appreciate all help!

1. Determine possible Maximum & minimum points and increasing & decreasing intervals, 2x²+3x²-36x+5, the answer is Max(-3,86) Min(2,-39), inc (-infinity, -3)U(2, infinity), dec (-3,2)

2. Determine possible Maximum & minimum points and increasing & decreasing intervals

3x⁴-4x³-36x²-5 , the answers are max(0,-5), min (-2,-69) and (3,-194), inc(-2,0)U(3, infinity), dec (-infinity,-2)U(0,3)

3. Determine possible Maximum & minimum points and increasing & decreasing intervals, x³-3x²-24x+2, max(-2,30), min (4,-78), inc(-infinity, -2)U(4,infinity), dec (-2,4)

4. Determine Possible Inflection Points & Intervals of Concavity x+4/x-4, the answer says no IP, concave up(4, infinity), and concave down(-infinity, 4)

I have tried to solve these but I got the wrong answers, so if you could please give me show me how y'all got the answer. I'm not sure what I did wrong, but I will compare and contrast what I did wrong to figure out the correct way to solve problems for next time. Thank you!