I have an ellipse problem, (ellipse centre is (0,0)) I am given a value of it's eccentricity 1/7 √39, along with the identity e = √(1 - (b^2/a^2)) & asked to show how to get from one to the other.

Obviously I get e = 1/49 √39

(or e = √39/49)

by substituting a (7) & b (√10)

I don't have the identity √((a^2 - b^2) / a) which would solve this, but could anyone suggest how I could go about doing this, please?