Directly opposite it on the fixed scale is a new tick mark, which represents a third number. That third number is the product of the first two. A similar method allows one to divide. Of course not all the tick marks have labels, just as they don't on most rulers, and you have to figure out what number is represented by what mark. Furthermore, many numbers do not have tick marks to represent them, so you may have to imagine a virtual tick mark between real ones.
Many slide rules have more scales than just the two described above.
Often there is a scale to allow squaring, which goes from 1 to 100. There is usually a reverse scale from 10 to 1 to compute reciprocals. There may be an evenly marked scale, like a ruler, which allows computing of logarithms. Some have more scales to deal with exponentials and trigonometric functions like sine, cosine, and tangent. Another feature of a slide rule was a transparent slide bearing a hairline mark perpendicular to the scales.