Resposta:
[tex]\left(3x+1\right)^2\\\\\left(3x\right)^2+2\cdot \:3x\cdot \:1+1^2\\\\\left(3x\right)^2+2\cdot \:3x\cdot \:1+1^2=9x^2+6x+1\\\\=9x^2+6x+1[/tex]