First you do the multiplication (BEDMAS) so it's 36/4 which is 9. then you do 15-9 which is 6. after that you do the second equation which is 9x2 = 18. so 6+18= 24 :))
Each transversal crossing a line generates 2 pairs of vertical angles and for 3 transversal crossing the same line it will be 3 x2 = 6 pairs (or 12 angles)