First, subtract #color(red)(18)# from each side of the equation to isolate the #u# term while keeping the equation balanced:
#3u^2 + 18 - color(red)(18) = 99 - color(red)(18)#
#3u^2 + 0 = 81#
#3u^2 = 81#
Next, divide each side of the equation by #color(red)(3)# to isolate #u^2# while keeping the equation balanced:
#(3u^2)/color(red)(3) = 81/color(red)(3)#
#(color(red)(cancel(color(black)(3)))u^2)/cancel(color(red)(3)) = 27#
#u^2 = 27#
Now, take the square root of both sides of the equation to solve for #u# while keeping the equation balanced. Remember, the square root of a number produces a positive and negative result:
#sqrt(u^2) = +-sqrt(27)#
#u = +-sqrt(9 * 3)#
#u = +-sqrt(9)sqrt(3)#
#u = +-3sqrt(3)#
