Find the value of 'n'.

First you cross multiply
n x (n+1) =210
Then you open up the brackets
n^2 +n=420
n^2 + n -420 =0
Because -20 x 21= -420
And 21-20=1, 21 and -20 have become the roots of this equation.
(n^2 + 21n)(-20n -420)=0
Factorise
n(n+21) -20(n+21)
(n+21)(n-20)
n+21=0, n-20=0
n=-21, n=20

Use a calculator basically