The line with equation $y=4x+c$ is a tangent to the curve with equation $y=x^2-x-5$. Find the value of $c$.
I did it
$y=x^2-x-5$
$4=2x-1$
$\frac{5}{2}=x$
$x=\frac{5}{2}$
$=(\frac{5}{2})^2-\frac{5}{2}-5$
$=-\frac{5}{4}$
I got $-\frac{5}{4}$ but the right answer is $-\frac{45}{4}$
Help me out!