The Diameter of a circle is basically the longest line that you can draw in it, which goes through the origin/center.
It's also 2x the radius of a circle (the origin/center to any point on the circumference)
I would post a picture of it but I still have no idea how to do that (and I'm too lazy to check the stickied topic)
You can use it to find the circumference of a circle. where d=diameter, C=circumference
You didnt specify what you wanted help with, but I assume you would like help simplifying it?
According to BEDMAS (brackets, exponents, division, multiplication, addition, subtraction), we should first simplify the brackets bit "-2(-4x+5)"
Now we can add the 6x
So we get the final answer of