



 
I think you are looking at this diagram: Remember that radii meet points of tangency at right angles, so that means the two radii drawn are parallel to each other. Now if I draw a blue line parallel to the tangent line here: Then I have created a rectangle. Therefore the length of the blue line is the same as the length x you are trying to find. As well, you know the lengths of two of the sides of the triangle you see there and can show that it is a right triangle: Solve for x. Cheers,  


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