Question 94457
We'll use L and W for this as in length and width.

{{{L=3x-4}}}
{{{W=7x+10}}}

To find perimeter, use {{{P=2L+2W}}}
{{{P=2(3x-4)+2(7x+10)}}}
{{{P=6x-8+14x+20}}}
{{{P=20x+12}}}

That should do it, for you.