If the length of FH is 18 units, what is the value of x?

we know that
[tex]FH=FG+GH[/tex]
In this problem we have
[tex]FH=18\ units\\FG=4x\\GH=2x[/tex]
substitute in the formula above
[tex]18=4x+2x[/tex]
solve for x
[tex]6x=18[/tex]
[tex]x=18/6[/tex]
[tex]x=3\ units[/tex]
therefore
the answer is
The value of x is [tex]3\ units[/tex]