I have 3 vectors a = 2i - j + k, b = i -j +k, c= -i - 2j + 2k

I have to find:

1) the value of w when a + wb is orthogonal to c

2) The value(s) of w that make c at 45 degrees to -i + wk

The answers are:

1) 4/3

2) 1,7

I have tried this and no matter what I do I cannot get these answers. Please help. Though to be honest I'm not sure where to start other than:

x1x2 + y1y2 + z1z2 = 0

Can anyone help?