local monitorprintcentred = function(text, side)
local monitor = peripheral.wrap(side)
local maxw, maxh = monitor.getSize() --screen dimensions
local curx, cury = monitor.getCursorPos() --cursor pos

monitor.setCursorPos((maxw-#text)/2,cury) --set it to the point where write will make the text centred
monitor.write(text) --write the text
monitor.setCursorPos(curx,cury+1) --return the cursor to the same position on the next line
end

local monitor = peripheral.wrap("left") -- makes 'monitor' refer to the monitor on the left side
local maxw, maxh =monitor.getSize() --screen dimensions
monitor.setCursorPos(1,math.floor(maxh/2))--set cursor to the start of the upper middle line

monitorprintcentred("ATM","back")--print centred on that line