The formula of a midpoint:
[tex]M\left(\dfrac{x_1+x_2}{2};\ \dfrac{y_1+y_2}{2}\right)[/tex]
The formula of a distance between two points:
[tex]d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}[/tex]
We have the points P(3, 5) and Q(7, 5). Substitute:
[tex]x=\dfrac{3+7}{2}=\dfrac{10}{2}=5\\\\y=\dfrac{5+5}{2}=\dfrac{10}{2}=5[/tex]
[tex]d=\sqrt{(5-5)^2+(7-3)^2}=\sqrt{0^2+4^2}=\sqrt{4^2}=4[/tex]