If anyone is wondering, slide rules use logarithms to turn addition into multiplication and subtraction into division. So think about how you would do addition with two rulers, if you were adding 2+3 you would line up the rulers so that the 0 on one was at the 2 on the other. Then you would see that the 3 on that one would be lined up with the 5 in the first ruler. If you follow that procedure in a slide rule then you will have done multiplication instead.
If anyone is wondering, slide rules use logarithms to turn addition into multiplication and subtraction into division. So think about how you would do addition with two rulers, if you were adding 2+3 you would line up the rulers so that the 0 on one was at the 2 on the other. Then you would see that the 3 on that one would be lined up with the 5 in the first ruler. If you follow that procedure in a slide rule then you will have done multiplication instead.