If the two roots of the quadratic $4x^2+7x+k$ are $\frac{-7\pm i\sqrt{15}}{8}$, what is $k$?
I notice that the roots are complex numbers, so they must come from using the quadratic formula with a negative discriminant. That means the discriminant, which is $7^2-4(4)k$, must be equal to $-15$. Solving for $k$, I get $k=\frac{7^2+15}{4(4)}=\frac{64}{16}=4$. 