the problem is that I have to find the length of x.

Here's what they say:
Find the missing length, when the perimeter is 9x+11

then here are the length that are given on the shape: 2x+3 and 3x+1

How could I solve this?

Three sides: add them to get perimeter.

x + 2x+3 + 3x+1= 9x+11

solve for x.

But the problem is about adding and subtracting polynomials...

This is what I did...

2x+3+3x+1
=2x+3x+3+1
=5x+4

and

9x+11-5x+4
=9x-5x+11+4
=4x+15

but then, if I add both 5x+4+4x+15
=9x+19...

BobPursely gave this equation:

x + 2x+3 + 3x+1= 9x+11

If you only look at the left side, you'll get 6x + 4.

Your new equation should read:

6x + 4 = 9x + 11

Does that help?

im not sure?

Ok. From here, you need to solve for x. The easiest way to do that is to get x on one side of the equation. Do you know how to do that?

yes :)

To find the missing length, we need to use the perimeter equation. Let's break down the problem.

Perimeter is the sum of all sides of a shape. In this case, the perimeter is given as 9x+11. This means that when we add up all the sides of the shape, it is equal to 9x+11.

We are given two of the lengths: 2x+3 and 3x+1. Let's assume these two lengths are the sides of a rectangle, for example.

In a rectangle, opposite sides are equal in length. So if we assume 2x+3 is one side of the rectangle, then the opposite side would also be 2x+3.

Similarly, if 3x+1 is one side of the rectangle, the opposite side would also be 3x+1.

Now, let's add up the lengths of all sides of the rectangle to obtain the perimeter:

(2x+3) + (3x+1) + (2x+3) + (3x+1) = 9x+11

Simplifying the equation, we get:

2x + 3 + 3x + 1 + 2x + 3 + 3x + 1 = 9x + 11

Combine like terms:

10x + 8 = 9x + 11

Now, we need to isolate the variable x. To do this, we can subtract 9x from both sides of the equation:

10x - 9x + 8 = 9x - 9x + 11

Simplifying the equation further:

x + 8 = 11

Lastly, we can subtract 8 from both sides of the equation to solve for x:

x + 8 - 8 = 11 - 8

Simplifying:

x = 3

Therefore, the missing length, denoted as x, is equal to 3.