Answer
None of the equations are linear. See below for explanations.
Work Step by Step
(a) $x(x+1)=6$ is the same as $x^{2}+x=6$. This is not linear because linear equations can't have powers of $x$ greater than 1.
(b) $\sqrt{x+2}=x$, which is not linear because linear equations can't have powers of $x$ of 1/2.
(c) $3x^{2}-2x-1=0$. This is not linear because linear equations can't have powers of $x$ greater than 1.
