Step-by-step explanation:
[tex]3 {x}^{2} + 12x = 15 \\ 3 ({x}^{2} + 4x) = 15 \\ {x}^{2} + 4x = 5 \\ {x}^{2} + 4x = 5 \\ {x}^{2} + 4x + 4 = 5 + 4 \\ {x}^{2} + 4x + 4 = 9 \\ {x}^{2} + 4x + {2}^{2} = {3}^{2} \\ \red{ \bold{ (x + 2)^{2} = {3}^{2}}} \\ [/tex]
