Question

x^2 +3x . (x-12 )=0

Answer 1

x^2 +3x . (x-12 )=0
x^2 + 3x^2 - 36x= 0
4x^2 - 36x = 0
4x (x - 9) = 0
x = 0

x - 9 = 0
x = 9

S = { 9 ; 0 }

Answer 2

x² + 3x.(x-12)=0
x²+3x²-36x=0
4x²-36x=0

a = 4  b= -36  c=0

Δ=b²-4.a.c
Δ=-36² -4 × 4 x 0
Δ= 1296

Bhascara 
-b+-Δ/2.a
36 + 36 /8 = 9
36-36/8= 0

x'=9  x"=0