What is the correct measure of an astronomical unit?

In our system an astronomical unit is the average distance from the earth to the sun which is about 93 million miles. (92,957,000 miles) This is known as 1 AU.

Distance^2= yellow line ^2 (see picture) = 30^2 + 49^2so distance = squareroot of (300^2 +49^2)i dont know if this is how to solve it, but i cnt think of another way