Are - 3.9 and - 0.99 on the same side or the opposite side of zero?

Same side. Locate - 3.9 on the number line. Then - 0.99 is to the right of - 3.9, with both numbers being on the same side (that is, on the left side) of zero.

Same side, think of it as a number line. there is a left side for negitives, and a right for positives. Every negitive number is on the same side, while the positives have their own side too. Only if it was a positive with a negitive it would be on opposite sides.