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2 Answers

n^2=18n + 40
n^2 - 18n - 40=0
n^2 -20n + 2n - 40=0
n(n - 20) + 2(n - 20)=0
therefore;
(n + 2) (n- 20)=0
Hence n = -2,20
n ^2 = 18n + 40
 
n ^2 - 18n - 40 = 0
 
n^2 + 2n - 20n - 40 =0 
 
n ( n +2 ) - 20 ( n + 2 ) = 0
 
( n + 2 ) ( n - 20 ) = 0
 
 n + 2 = 0 , n = - 2    n -20 = 0 ,n = 20