What are two ratios equivalent to 3: 11

There are a few ways to do this

When doing a ratio, its a comparison of numbers which often is was multiplied and then compared

So since you can't find a common factor above one to make it smaller, you have to multiply instead.

You can multiply each side by 3 and it would be 9:33 and it would be equivalent to
 3:11 if you divide each side by 3

Other examples:

Multiplying each side by 5⇒ 15: 55

Multiplying each side by 10⇒30:110