Hi there!
We can find the perimeter of a triangle by adding all of its sides.
The perimeter of the triangle in the picture is therefore as follows:
Now do some simplification. First work out the parenthesis.
Now rearrange our expression (in order to collect the terms later).
Finally collect the terms.
The second part of the question tells us that x equals 10. Therefore, we can plug in this value of x into our expression.
Hence, the perimeter of the triangle is 230 millimetres.
~ Hope this helps you!