New vector from known angle

Hi everyone, hoping I could get some Java specific help with my rusty maths knowledge.
I drew a little diagram to illustrate my problem:

So I have a ball moving along vector a, it hits the top bar and assuming I can calculate the angle of c, how would I calculate the new vector b for the ball to follow?

Thanks in advance for your help, I know this is is something really simple but I’ve been stuck for a while :frowning: