Explanation:
draw 3 boxes.
put 6 on the side of the one on the left and then put 3 over the first two boxes and then put one over the last box. then on the side of the last box draw three tiny ones.
i'll try and draw it
3 3 1
6 (18) (18) (6) then draw three tiny boxes.
you do this because 6 times 3 is 18 and if you do that twice its 36. then you do 6 times 1 which gives you 6. and 6 +36=42. then you add three boxes to make 45.
this probably made no sense. sorry