Step-by-step explanation:
[tex]\text{Given:}\\\\AB=3x-4\\AC=2x+12\\BC=7x-2\\AB:AC=1:2\Rightarrow(3x-4):(2x+12)=1:2\\\\\dfrac{3x-4}{2x+12}=\dfrac{1}{2}\qquad\text{cross multiply}\\\\(3x-4)(2)=(2x+12)(1)\qquad\text{use the distributive property}\\\\(3x)(2)+(-4)(2)=2x+12\\\\6x-8=2x+12\qquad\text{add 8 to both sides}\\\\6x-8+8=2x+12+8\\\\6x=2x+20\qquad\text{subtract}\ 2x\ \text{from both sides}\\\\6x-2x=2x-2x+20\\\\4x=20\qquad\text{divide both sides by 4}\\\\\dfrac{4x}{4}=\dfrac{20}{4}\\\\x=5[/tex]
[tex]\text{Put the value of}\ x\ \text{to}\ AC\ \text{and}\ BC:\\\\AC=2(5)+12=10+12=22\\\\BC=7(5)-2=35-2=33\\\\AC:BC=22:33=\dfrac{22}{33}=\dfrac{22:11}{33:11}=\dfrac{2}{3}=2:3[/tex]
