Cost of equity

IPA co is about to pay $0.50 dividend on each ordinary share. EPS was $1.50.
Net assets per share is $6. Current share price is $4.50.
What is the cost of equity?

I don’t understand how the growth is worked out.
G = rb
r = rate of return co can earn re-investment (EPS/Net asset per share)
b= proportion of retained earnings

Could you explain how r and b is worked out.
Thank you in advance!