15. (12 points) Given $b-a=\frac{1}{8}, 2 a^{2}+a=$ $\frac{1}{4}$. Find the value of $\frac{b}{a}-a$.
Three, 15. Multiply the equation $b-a=\frac{1}{8}$ by 2 and subtract the equation $2 a^{2}+a=\frac{1}{4}$, to get $2 b-2 a^{2}=3 a$.
Given that $a \neq 0$.
Divide both sides by $2 a$, to get $\frac{b}{a}-a=\frac{3}{2}$.