Write a linear expression in simplest form to represent the perimeter of the triangle at the right then find the perimeter of the value x is 10 millimeters

Seems like you forgot to add the diagram of the triangle.
However, I will tell you how to solve this and you can apply using the triangle you have.
For the first part:
To get the perimeter, all you have to do is sum up the three given side lengths.
For the second part:
You have the expression of the perimeter from the first part. Now substitute with x = 10 in the expression to get the value of the perimeter

Example:
Suppose that we have a triangle whose side lengths are as follows:
AB = 2x+3
BC = x-9
CA = x
For part a:
perimeter = AB + BC + CA
perimeter = 2x+3 + x-9 + x
perimeter = 4x - 6 units
For part b:
At x = 10:
perimeter = 4x - 6
perimeter = 4(10) - 6
perimeter = 34 mm

Hope this helps :)