Vector a has length 2 units and is directed to the north. vector b has length 5 units and is directed to the south. calculate the magnitude and direction of a-b

Just for convenience, let's say that North is the positive direction.
Then it must follow that South is the negative direction.

Vector A is a vector in the positive direction with a magnitude of 2 .

Vector B is a vector in the negative direction with a magnitude of 5 .

When they're subtracted, A-B is ( +2 ) - ( -5 ) 

                                                 =  ( +2 ) + ( +5 )

                                                 =       +7  .

The vector  (A-B)  points North and has a magnitude of  7 .