Let's note:
x=the grade in the final examination

We write the inequality which models the problem: \(\displaystyle\frac{{{86}+{88}+{x}}}{{3}}\Rightarrow{90}\)

We solve the inequality: \(\displaystyle{174}+{x}\Rightarrow{3}{\left({90}\right)}\)

\(\displaystyle{174}+{x}\Rightarrow{270}\)

\(\displaystyle{174}+{x}-{174}\Rightarrow{270}-{174}\)

\(\displaystyle{x}\Rightarrow{96}\)

We write the inequality which models the problem: \(\displaystyle\frac{{{86}+{88}+{x}}}{{3}}\Rightarrow{90}\)

We solve the inequality: \(\displaystyle{174}+{x}\Rightarrow{3}{\left({90}\right)}\)

\(\displaystyle{174}+{x}\Rightarrow{270}\)

\(\displaystyle{174}+{x}-{174}\Rightarrow{270}-{174}\)

\(\displaystyle{x}\Rightarrow{96}\)