Co-ordinate Geometery urgent help needed

A, B, C and D are (-7,4), (3,-1), (6,1) and (k,-15) respectively.

1] Find the gradient of AB

2] Find the equation of AB and simplify the answer

3] Find the length of AB

4] The point E is the mid-point of AB. Find the co-ordinates of E.

5] CD is perpendicular to AB. Find the value of k in D.

For question 1 I used y2-y1 over x2-x1 to get the gradient which I think is -6,

For question 2 I then used formula of a line and continued:

y - y1 = m (x - x1)

y - 4 = -6 (x + 7)

y - 4 = -6x - 42

y + 6x + 38 = 0

For question 3 I worked the length of AB to be:

√ (x2-x1)² + (y2-y1)²

√ (3 + 7)² + (-1 + 7)²

√ 10² + 6² [100 = 36]

AB = √136

I'm really concerned this is wrong and I don't know how to do the rest of the question. Can someone please help me?