If we reject the null hypothesis, h0: ρ = 0 , what can we conclude about the population correlation coefficient?

If the null hypothesis,  is rejected, then we can conclude that the correlation coefficient is significant. This means that there is enough evidence to conclude that a relationship exists between the two (or more) variables involved in the regression. 

It would be 15 m how i got the answer: (20x20) + b^2 = (25x25)400 + b^2 = 625625-400=225square that and get 15hope this : )