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So the quadratic equation is 
4n^2-81=0 \\ 4n^2=81 \\ n^2= \frac{81}{4}

We can have two values for n which is either the positive or negative squareroot of 81/4
n= \frac{9}{2}  \\ n=- \frac{9}{2}

We can also factor is out:
4n^2-81=(2n+9)(2n-9) and we get the same answer