68. $(5 a+2 b) x^{2}+a x+b=0$ is a linear equation in one variable $x$, and it has a unique solution, then $x=$
Answer: $\frac{5}{2}$.
Solution: Since the equation $(5 a+2 b) x^{2}+a x+b=0$ is a linear equation in one variable $x$, and has a unique solution,
we have
$$
5 a+2 b=0 \text {, and } a \neq 0 \text {. }
$$

Therefore,
$$
a=-\frac{2}{5} b, \text { and } b \neq 0 \text {. }
$$

Substituting $a=-\frac{2}{5} b$ into $a x+b=0$, we get
$$
-\frac{2}{5} b x+b=0
$$

Solving for $x$, we obtain
$$
x=\frac{5}{2} \text {. }
$$