If you are trying to solve for r you can use the quadratic equation to solve this problem:
The quadratic formula states:
For color(red)(a)x^2 + color(blue)(b)x + color(green)(c) = 0, the values of x which are the solutions to the equation are given by:
x = (-color(blue)(b) +- sqrt(color(blue)(b)^2 - (4color(red)(a)color(green)(c))))/(2 * color(red)(a))
Substituting:
color(red)(6) for color(red)(a)
color(blue)(1) for color(blue)(b)
color(green)(-12) for color(green)(c) gives:
r = (-color(blue)(1) +- sqrt(color(blue)(1)^2 - (4 * color(red)(6) * color(green)(-12))))/(2 * color(red)(6))
r = (-1 +- sqrt(1 - (-288)))/12
r = (-1 +- sqrt(1 + 288))/12
r = (-1 +- sqrt(289))/12
r = (-1 - 17)/12 and r = (-1 + 17)/12
r = -18/12 and r = 16/12
r = -3/2 and r = 4/3