We have a triangle with angles: 66 deg, 57 deg and (3x+21).
We have to solve for x.
We know that the sum of the interior angles of a triangles is 180 degrees.
Then, we can write and solve for x as:
[tex]\begin{gathered} 66+57+(3x+21)=180 \\ 3x+21=180-57-66 \\ 3x+21=57 \\ 3x=57-21 \\ 3x=36 \\ x=\frac{36}{3} \\ x=12 \end{gathered}[/tex]The value of x is 12.
Answer: A) 12
