Draw a picture of the circles and the tangent. You should be able to form a right-angled triangle, use pythatogra's thm to solve for the tangent (longest side).
Two circles of radii 5 cm and 8 cm touch externally. Calculate the length of the common tangent.
Here Ive attempted it, creating two simultaneous equations for the external point and the centers of each circle. From there i attempted the cosine rule, but to no avail.
Thanks for help.