Given: ab is a tangent of x. ab = 70 and xd = 12. what is the measure of db?

Answer = 74

Since angle DAB is a right angle (because the line is a tangent), then you can use the pythagorean theorem to calculate the length of DB.

XD = 12 and since XD is the radius of the circle, then you know that doubling it gives you the diameter, AD, which is then equal to 24.

The pythagorean theorem says that a^2 + b^2 = c^2 so then 70^2 + 24^2 = (DB)^2.

4900 + 576 = (DB)^2. 
5476 = (DB)^2
DB = 74