So the coordinates of A and B are basically finding the mid-point of RS and ST.

1^{st } to find the mid-point u use the formula \((\frac{x_1+x_2}{2}, \frac{y_1+y_2}{2})\)

The mid-point of RS is,

\((\frac{0+2a}{2}, \frac{0+2b}{2})\)

\((\frac{2a}{2}, \frac{2b}{2})\)

\((a,b)\)

\((a,b)\) are the coordinates of A.

Now the midpoint of ST is,

\((\frac{2a+2c}{2}, \frac{2b+2d}{2})\)

\((\frac{2(a+c)}{2}, \frac{2(b+d)}{2})\)

\(((a+c),(b+d))\)

\(((a+c)+(b+d))\) are the coordinates of ST.

I didn't quite understand what the slope part means