To find the midpoint of the two points (x₁ ,y₁) and (x₂, y₂) we need to use the formula:
[tex]\sf{Midpoint=(\frac{x_1 + x_2}{2}, \frac{y_1+y_2}{2})}[/tex]
So to find the midpoint of E (a,a) and F (3a, a), let's plug it in to the formula:
[tex]\sf{Midpoint=(\frac{a + 3a}{2}, \frac{a+a}{2})}[/tex]
Simplify the numerator:
[tex]\sf{Midpoint=(\frac{4a}{2}, \frac{2a}{2})}[/tex]
Simplifying the fractions more:
[tex]\sf{Midpoint=(2a, a)}[/tex]
So the midpoint of EF is (2a,a).
Your answer of (a,a) would be wrong.