What is the measure of ac ? enter your answer in the box. ° circle with inscribed angle a b c. angle a b c is 3 x minus 1.5 degrees. the intercepted arc a c is 3 x plus 9 degrees.

Inscribed angles are 1/2 of the measure of the intercepted arc.  That means from this we have the equation
.  Distribute the 1/2:

We cannot have a variable on both sides of the equation, so cancel the 1.5x to avoid negatives:

Combine like terms:

Cancel the -1.5 by adding:

Divide both sides by 1.5:

Since AC is 3x+9, we substitute 4 in for x:
3*4+9=12+9=21°

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