All you need to do is be able to write an expression for the terms you are adding, then decide how many there are. The expression for the nth term will clearly be a fraction with numerator = 1, and with a denominator that is the product of two numbers whose difference is 1. If you want to start with n = 1, the two factors in the denominator are then (n+2) and (n+1). Finally, there are 7 terms in the sum, so ...
7
Σ. (1/((n+2)(n+1))
n=1
Alternately, start with n = 3:
9
Σ 1/((n)(n-1))
n=3
