Should be:...do you know what that is? Well, to be honest, it's pretty long winded and I don't want to get into it, so I'll just give you the formula:
a = cos(angle)
b = -sin(angle)
c = sin(angle)
d = cos(angle)
There you have it. That will rotate your object by whatever you specify "angle" to be. You'll notice it looks a lot like what you wanted it to, doesn't it? You skew it by the same amount along each axis, with one of them flipped to negative, then you scale it to make sure it's the same size again. If you want to rotate the other way, just make b negative and c positive instead.
just make c negative and b positive instead.