For science, we have to make a steel plate float in fresh water without adding anything to the water and changing the mass of the steel plate. You can't do anything to change the steel plate and we have to solve it mathematically. I've already tried making proportions with the steel plate and water, finding out the densities of steel and fresh water, and trying to add more volume to make it bigger. What do I do?

You sure you can't do ANYTHING to the steel? You say you can't do anything but later say you tried adding more volume. That's change.

The obvious thing would be to bend it into a boat shape. That doesn't change it's mass. It adds air to the area displaced by the slightly sunken part below the water line. The other would be to simply use a tiny thin piece of steel, so thin it would for a while be held by surface tension.
Sorry I don't know the math.

Very correct.. changing its shape will do the trick.

Freeze the water ;)

You can ask yourself why a ship made of steel does not sink? And the answer is that the density within the steel ship is not equivalent to the density of water around and the weight of steel is not equal to the weight of water in equivalent area.