Please help. thank you

Hi!

So, first, let's just form the equation:

We know that all the side lengths equal 12 cm.

So the equation will be:

5 + 3 + x = 12

The answer you put in the box is:

5 + 3 + x

To find the value of x, we just solve this equation:

5 + 3 + x = 12

Combine like terms:

8 + x = 12

Same to both sides:

x = 12 - 8

x = 4

I hope this helped, let me know if you have any questions.