If N is midpoint of QR, then you must use the formula of a midpoint:
[tex]M\left(\dfrac{x_1+x_2}{2},\ \dfrac{y_1+y_2}{2}\right)[/tex]
We have Q(0, 2b) and R(2c, 0). Substitute:
[tex]x=\dfrac{2c+0}{2}=\dfrac{2c}{2}=c\\\\y=\dfrac{2b+0}{2}=\dfrac{2b}{2}=b[/tex]
Answer: (c, b)
