first, group them in 2's (2 numbers per group) and see which ones have the factors of each other
you want to group 12x^3 with 3x because 3 is a factor of 12 then you want to group -20x^2 and -5 together becuase 5 is a factor of 20 so (12x^3+3x)+(-20x^2-5) undistribute (3x)(4x^2+1)+(-5)(4x^2+1) revese distribute (ab+ac=a(b+c)) (3x-5)(4x^2+1)
