Showing text in the middle of monitors (Advanced of course)

So far I have this:

m = peripheral.wrap("back")

m.clear()
mX,mY = m.getSize()
x = math.floor(mX/2) - math.floor(string.len("text")/2)
y = math.floor(mY/2) - math.floor(string.len("text")/2)
m.setTextScale(1)
m.setCursorPos(x, y)
m.write("Market")

Please read this, specifically the section about posting a good question. You've left us to guess what you expect the program to do and what it's actually doing instead. Also, you haven't asked a question.

In your case, the problem is obvious - you're setting the cursor position based on the length of the string "text", then you're writing the string "Market". "Market" will never be centered as long as you position the cursor based on the string "text" instead of the string "Market".

What's your problem?

This should work:

local width, height = term.getSize()
local function centreWrite( text )
	term.setCursorPos( math.floor( width / 2 - #text / 2 ) + 1, math.floor( height / 2 ) + 1 )
	term.write( text )
end

Just replace term with 'm' or whatever.