B1= r * (sy/sx)

how do i get the number for r (correlation) given the averages of x and y and their standard deviations?

In getting the correlation of R where the given is the average of X and Y and calculate their standard deviation. So correlation coefficient is a number that quantities some type of correlation and dependence and the standard deviation are a quantity calculated to indicate the extent of deviation for a group as a whole.
hope it helps