Given that the total sum of interior angles of a triangle = 180 degrees.
Thus, we have:
(3x + 21) + 66 + 57 = 180
Let's find the value of x.
3x + 21 + 66 + 57 = 180
3x + 144 = 180
Subtract 144 from both sides:
3x + 144 - 144 = 180 - 144
3x = 36
Divide both sides by 3:
[tex]\begin{gathered} \frac{3x}{3}=\frac{36}{3} \\ \\ x\text{ = 12} \end{gathered}[/tex]
ANSWER:
12
