Help with 7th grade math: scale factors!!

Okay well in class we have been working on scale factors. We've been doing this for several days now an i dont understand at all because im in the back of the room and i always forget my glasses for class (my contacts arent arriving untill next week) and i need help!
I know that the scale factor from a small shape to a big shape is just how many times the side length has to be multiplied to get to the side length of the larger figure. Example: triangle B is 3inches in length by 3inches in height. Triangle A is 6inches in length by 9inches in height. The scale factor from B to A is 3.
But my question is how to get the scale factor from figure A to figure B. Like, how do u get a scale factor if the images/shape/figure is getting smaller? So how to find the scale factor from a big shape to a small shape? PLEASE help.
Thanks in advanced. Ps my homework is due tomorrow.
Thanks!!!

Same deal. The scale factor is what you multiply the original by to get the scaled shape.

If the scale factor is greater than 1, the scaled shape will be larger.

If the scale factor is less than 1, the scaled shape will be smaller.

If the scale factor from shape A to B is B/A, then the scale factor from shape B to A is A/B, the reciprocal of the other scale factor.

If the scale factor in one direction is 3, in the other direction it will be 1/3.