Answer:
[tex]x=21[/tex]
Step-by-step explanation:
We have been given that point B is the midpoint of AC. This means AB will be equal to BC and AC will be two times AC.
[tex]AC=AB+BC..(1)[/tex]
[tex]AB=BC..(2)[/tex]
Using substitution, we will get:
[tex]AC=AB+AB[/tex]
[tex]AC=2AB[/tex]
Substitute given values.
[tex]5x+21=2(3x)[/tex]
[tex]5x+21=6x[/tex]
Switch sides:
[tex]6x=5x+21[/tex]
[tex]6x-5x=5x-5x+21[/tex]
[tex]x=21[/tex]
Therefore, the value of x is 21.
