The important parts (showing the base case and formulating the induction step) are complete! All that's left is doing some algebra, which I won't bore you with. However, if you're interested the steps are listed below:
We get n(n+1)/2 + n+1= (n2+3n+2)/2=(n+2)(n+1)/2=((n+1)+1)(n+1)/2. The right hand side of the equation is the equation of the statement but with n+1 substituted for n. Therefore we have shown that the relation holds for n+1; the inductive step is complete, and with the base case we show that it is true for all natural numbers n