Question 111940
Let's see. Perimeter is the distance around the plane.
So, we just add the sides:
{{{ (4x + 1) + (7x - 2) + (7x + 1) }}}
And we combine like terms:
{{{ (4x + 7x + 7x) + (1 + (-2) + 1) }}}
And add:
{{{ 18x + 0 }}}
Remove the zero:
{{{ 18x }}} 
And there's your final answer!

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-WhizKid