The question asks me to determine the intersection 3x-y=11 and 3x^{2}-y^{2 }= 47 This is the given answer.

y=3x-11

3x^{2}-(3x-11)^{2}=47

3x^{2}-9x^{2 }+66x-121-47=0

-6x^{2}+ 66x - 168= 0

x^{2 }-11x +28 = 0

..... and so on

The problem I'm having is in line 3. I'm not sure where the '66x' came from?