If a and b are the non-zero roots of the equation x² + ax + b = 0, then the least value of the expression x² + ax + b is

Given a, b are the roots of the equation $x^2+ax+b=0$

Sum of the roots is $a+b=-a$ and product of the  roots is $ab=b$

it implies that $a=1,b=-2$

Hence, the given expression is $x^2+x-2=0$

Since  $a=1>0$ it has maximum value , and that value is $\frac{4ac-b^2}{4a}=\frac{4\left(-2\right)-1}{4}=\boxed{-\frac{9}{4}}$