Answer :
The formula to find the distance between two points A(x₁,y₁) and B(x₂,y₂) is:
[tex]D=\sqrt[]{(x_2-x_1)^2+(y_2-y_1)^2}[/tex]In this case, we have:
[tex]\begin{gathered} B(x_1,y_1)=(1,-7) \\ C(x_2,y_2)=(1,-4) \end{gathered}[/tex][tex]\begin{gathered} D=\sqrt[]{(1-1)^2+(-4-(-7))^2} \\ D=\sqrt[]{(0)^2+(-4+7)^2} \\ D=\sqrt[]{3^2} \\ D=3 \end{gathered}[/tex]Therefore, the length of segment BC is 3 units.