As a general rule, the best solutions to review begin with 'a few steps' (use the 'Options' tab) but your inquiry can be more fully explained by selection 'a lot of steps'.  That solution process will show clearly how "+11" is being added to both sides and "-6u" is being added to both sides, hence that is expressed as "(11-6u)" added to both sides... the intent is to transfer all numerics to the rightside and all variables to the leftside, from which 'u' is solved for.  It is also useful to click on a particular solution step in order to review the logic which is employed.