\(a)\left(x+6\right)^2-x\left(x+3\right)=5\\ \Leftrightarrow\left(x^2+12x+36\right)-x^2-3x=5\\ \Leftrightarrow x^2+12x+36-x^2-3x=5\\ \Leftrightarrow9x+36=5\\ \Leftrightarrow9x=5-36\\ \Leftrightarrow9x=-31\\ \Leftrightarrow x=-\dfrac{31}{9}\\ b)\left(x+7\right)\left(x-7\right)-\left(x-3\right)^2=x+1\\ \Leftrightarrow\left(x^2-49\right)-\left(x^2-6x+9\right)=x+1\\ \Leftrightarrow x^2-49-x^2+6x-9=x+1\\ \Leftrightarrow6x-58=x+1\\ \Leftrightarrow6x-x=1+58\\\Leftrightarrow 5x=59\\ \Leftrightarrow x=\dfrac{59}{5}\\ c)\left(x+2\right)\left(x-2\right)-\left(x+1\right)\left(x-3\right)=x-1\\ \Leftrightarrow x^2-4-\left(x^2-3x+x-3\right)=x-1\\ \Leftrightarrow x^2-4-x^2+2x+3=x-1\\ \Leftrightarrow2x-1=x-1\\ \Leftrightarrow2x-x=-1+1\\ \Leftrightarrow x=0\)
