Hey

I suppose it will be like

A vector into B vector is (3*12)+(4*5)=36+20= 56

AB= root under (3 square+4 square) * root under (12 sqaure + 5 square)= 5+13=18

Hence mod of cos theta will be : 56/18= 3.11(which is not possible hence I guess you got the question wrong and it should have been either -4j cap or -5 j cap(one of these)).