Correlation Coefficient

So I'm trying to do the proof for the correlation coefficient showing that:

corr(a1x + b1, a2x + b2)
a1a2>0 = corr(x,y)
a1a2<0 = - corr(x,y)

Can anyone walk me through this?

I thought I knew what I was doing but I'm not exactly sure how the given equation fits in.

Thanks.


Correlation Coefficient