Given $f(x,y)=3x^2+3xy+y^2+9x-7,$ find all points at which $\frac{\text{d}\text{e}\text{l}\text{f}}{\text{d}\text{e}\text{l}\text{x}}=0$ and $\frac{\text{d}\text{e}\text{l}\text{f}}{\text{d}\text{e}\text{l}\text{y}}=0$ simultaneously.