As you become more familiar with a slide rule, you might discover other shortcuts.
For example, here is another way to multiply 5 7:
Line up the 5 on the top scale with 10 on the bottom scale, then find the number on the top scale that is directly above 7. The answer you find is 3.5, which you then have to multiply by 10 to find the correct answer, which is 35.
Why does the shortcut work? Does it also work if you line up 7 with 10 and read off the number above 5?