You're left a lot of space with those instructions... I'm assuming that a is the length of one of the sides and that the "d"'s stand for the diagonals, or length between the corners of the shape. Basically make the sides 3cm long and the diagonals to add up to 7.5cm.

The only thing that you have to worry about is angling the sides correctly so that the diagonals add up to 7.5cm.