Two circles of radii 5cm and 8cm touch each other externelly,calculate the length of the common tangent.

and

if the radii of two intersecting circles are 51cm and 74cm and the length of their common chord is 48cm

A) Calculate the length of the line joining their centres

B) Calculate the length of the common tangent

In both questions i get them all set up draw them out right, i just can't seem to figure out what the common tangent length is (mathematically and literally) maybe there is a rule i am forgetting?

thanks for the help