To find the perimeter of any figure, we add up the lengths of the sides.

So we add up the measures of the sides by putting a + sign between
the expressions for the sides.

 2x³-3x²+8  +  3x²-6x-5  +  7x³-10x+5

We close up the spaces

 2x³-3x²+8+3x²-6x-5+7x³-10x+5

We pick out the terms that have x³, which are 2x³+7x³, 
combine them, get 9x³, so we take them away and put in 9x³

 9x³-3x²+8+3x²-6x-5-10x+5

We pick out the terms that have x², which are -3x²+3x², 
combine them, get 0, so we just take them out.

 9x³+8-6x-5-10x+5

We pick out the terms that have x, which are -6x-10x, 
combine them, get -16x, so we take them away and put in -16x

 9x³+8-16x-5+5

Pick out the terms that are just numbers, which are +8-5+5, 
combine them, get +8, so we take them away and put in +8.

 9x³+8-16x, which is the answer.

That's the formula for the perimeter.  (The actual numerical 
value of the perimeter will vary depending on what somebody 
might choose for x, but all we are looking for is this formula 
for the perimeter.  Quite often we will be asked to find 
something, and what they really mean is for us to find a 
formula for it, as in this case).

Edwin