circles p and q have radii 6 and 2 and are tangent to each other. find the length of their common external tangent ab

Refer to the diagram shown below.

x = the length of the common tangent to the two circles.
From the Pythagorean Theorem,
x² + 4² = 6²
x² = 6² - 4² = 36 - 16 = 20
x = √(4)(5) = 2√5 = 4.472

Answer: 2√5  or  4.472