## correlation coefficient between parameter estimates

Assuming a linear function y=a+xb. I have been given the parameters estimates which are are: a=11.7 and b=.95 ; also given is the (X'X)^-1
2x2 matrix=[.4 -2]
[ -2 12]
what is the estimated correlation coefficient between the parameter estimates at this stage?

I'm guessing that this is more of a question about the concept that needs to be understood about the X'X matrix.