The line with equation y=4x + c is a tangent to the curve with equation y=x^2-x-5. The value of c is?
The gradient of the tangent is 4. Therefore solve dy/dx = 4 for x - this is the x-coordinate of the point of tangency. Use it to get the y-coordinate of the point of tangency.
So now you know the coordinates of the point of tangency and this point lies on y = 4x + c .....