Answer the following (internal choice):
a. Two friends while flying kites from different locations, find the strings of their kites crossing each other. The strings can be represented by vectors a=3i^+j^+2k^ and b=2i^−2j^+4k^ . Determine the angle formed between the kite strings. Assume there is no slack in the strings.
b. Find a vector of magnitude 21 units in the direction opposite to that of AB where A and B are the points A(2, 1, 3) and B(8, -1, 0) respectively.