First you have to find the factors that 24 and 144 have in common. The factors must equal to 24 and also multiply to 144. The factor they have in common is 12. 12+12=24 and 12 * 12 =144.
Next, just substitute the numbers in the equation and you get.
(x-12)(x-12)
Simplify.
C. (x-12)^2
If you did distributive property, you would get the original equation.