First, add #color(red)(84)# to each side of the equation to isolate the #j# term while keeping the equation balanced:
#3j^2 - 84 + color(red)(84) = 0 + color(red)(84)#
#3j^2 - 0 = 84#
#3j^2 = 84#
Next, divide each side of the equation by #color(red)(3)# to isolate #j^2# while keeping the equation balanced:
#(3j^2)/color(red)(3) = 84/color(red)(3)#
#(color(red)(cancel(color(black)(3)))j^2)/cancel(color(red)(3)) = 28#
#j^2 = 28#
Now, take the square root of each side of the equation to solve for #j# while keeping the equation balanced. Remember, the square root of a number gives both a positive and negative result:
#sqrt(j^2) = +-sqrt(28)#
#j = +-sqrt(4 * 7)#
#j = +-sqrt(4)sqrt(7)#
#j = +-2sqrt(7)#
