The first formula we will use here is, well, Pythagoras' Theorem. We use it to find the length of the other side, and then use the half-base*height formula to find the area. Here goes:

Using a^2=b^2+c^2 (where a is the length of the hypotenuse, b,c the length of the other two sides)

=> a^2 - b^2 = c^2

using a=47.3, b=30.8, we get

47.3^2 - 30.8^2 = c^2

1288.7 = c^2

squareroot(1288.7) = c

=> c = 35.9

For area:

0.5*30.8*35.9 = 552.8 cm^2