start writing the corresponding equation using the corresponding formula for the average grade
[tex]\begin{gathered} \bar{x}=\frac{x_1+x_2+x_3}{3} \\ \text{then, } \\ 84=\frac{75+81+x_3}{3} \\ 84=\frac{156+x_3}{3} \end{gathered}[/tex]solve the equation
[tex]\begin{gathered} 84\cdot3=156+x_3 \\ 252=156+x_3 \\ 252-156=x_3 \\ 96=x_3 \end{gathered}[/tex]He must get at least 96 points on his test to keep an average of 84
