First set the equation to zero by adding -400 to both sides.
[tex]16 {r}^{2} - 400 = 0[/tex]
Next factor 16
[tex]16 ({r}^{2} - 25)= 0[/tex]
Divide through by 16
[tex] \frac{16 ({r}^{2} - 25)}{16} = \frac{0}{16} [/tex]
[tex] ({r}^{2} - 25)= 0[/tex]
This is difference of two squares
[tex](r - 5)(r + 5) = 0[/tex]
[tex]either \: r - 5= 0 \: or \: r + 5= 0[/tex]
[tex]r = 5 \: or \: - 5[/tex]
