First, subtract #color(red)(13)# from each side of the equation to isolate the #n# term while keeping the equation balanced:
#-color(red)(13) + 13 - 8n^2 = -color(red)(13) - 1139#
#0 - 8n^2 = -1152#
#-8n^2 = -1152#
Next, divide each side of the equation by #color(red)(-8)# to isolate #n^2# while keeping the equation balanced:
#(-8n^2)/color(red)(-8) = (-1152)/color(red)(-8)#
#(color(red)(cancel(color(black)(-8)))n^2)/cancel(color(red)(-8)) = 144#
#n^2 = 144#
Now, take the square root of each side of the equation to solve for #x# while keeping the equation balanced. Remember, the square root of a number produces both a positive and a negative result:
#sqrt(n^2) = +-sqrt(144)#
#n = +-12#