How to work out area of smaller triangle? Picture included

The area of the larger triangle is 5.6cm^2. I don't know how to work out the smaller one.

I know you have to use this sign- 1/2 a b sin C

But I don't know how to get C

http://plixi.com/p/94442507

Thanks in advanced :)

sides of big triangle: sides of small triangle = 4 : 3
area of small triangle = area of big triangle x (3/4)^2 = 5.6 x 9 / 16 cm^2 = 3.15 cm^2

area of smaller triangle
= 5.6 * (3/4)^2
= 3.15 cm^2