Let
[tex]A=(A_x, A_y),\quad M=(1,5), B=(4,7)[/tex]
By definition, the coordinates of the midpoint are the average of the coordinates of the endpoints:
[tex]M_x=\left(\dfrac{A_x+B_x}{2}\right),\quad M_y=\left(\dfrac{A_y+B_y}{2}\right)[/tex]
We deduce, solving for the coordinates of A:
[tex]A_x=2M_x-B_x,\quad A_y=2M_y-B_y[/tex]
Plug your values:
[tex]A_x=2\cdot 1-4,\quad A_y=2\cdot 5-7[/tex]
Which evaluates to
[tex]A_x=-2,\quad A_y=3[/tex]
