\(a)\) \(9{x^2} + 6xy + {y^2}\) \( = {\left( {3x} \right)^2} + 2.\left( {3x} \right)y + {y^2} = {\left( {3x + y} \right)^2}\)
\(b)\) \(6x - 9 - {x^2}\) \( = - \left( {{x^2} - 2.x.3 + {3^2}} \right) = - {\left( {x - 3} \right)^2}\)
\(c)\) \({x^2} + 4{y^2} + 4xy\) \( = {x^2} + 2.x.\left( {2y} \right) + {\left( {2y} \right)^2} = {\left( {x + 2y} \right)^2}\)
