Answer:
[tex]\huge\boxed{x=9}[/tex]
Step-by-step explanation:
[tex]\angle A\cong\angle B\qquad\text{therefore}\qquad AC\cong BC\Rightarrow|AC|=|BC|\\\\\text{We have}\ |AC|=20;\ BC=3x-7.\\\\\text{Therefore}\\\\3x-7=20\qquad|\text{add 7 to both sides}\3x-7+7=20+7\\\\3x=27\qquad|\text{divide both sides by 3}\\\\\dfrac{3x}{3}=\dfrac{27}{3}\\\\x=9[/tex]
