is 49 then you have two solutions because the solutions of $x=\frac{-b\pm7}{2a}$ using the quadratic formula and as you can see you have two solutions $\frac{-b+7}{2a}$ and $\frac{-b-7}{2a}$...to have only one solution you need your discriminant to be zero so the value of x that makes the polynomial zero is given by $x=\frac{-b\pm0}{2a}$ which just gives $\frac{-b}{2a}$ as your answer...and finally to have zero solutions you need the discriminant to be negative because your answer will be $\frac{-b\pm{ic}}{2a}$ where $i=(-1)^{\frac{1}{2}}$...and since $i$ is an imaginary number there are no REAL solutions