if you are asking about AB, Then in order to be a tangent to the circle the m(A) should be equal to 90 degrees. So we have to test it, if measure of angle "A" is 90 or not. Since the 3 sides lengths were given, I guess they want you to use Pythagorean theorem which says the following:
[tex] hyp^{2}= adj^{2}+opposite^{2}[/tex]
hypotenuse in here is the 16 unit length side, the adjacent is AB and the opposite is the diameter with length of 6.4 unit length.
if [tex] 16^{2} = 6.4^{2}+9.6^{2}[/tex] then m(A)= 90 degrees and it will be a tangent.
[tex]16^{2}=256[/tex]
[tex]6.4^{2} + 9.6^{2}=40.96+92.16=133.12 [/tex]
so the theorem is not applicable in this situation so the [tex]m(A) \neq 90 degrees[/tex]
therefore AB is not a tangent
