The first ste here is the find the general equation for the gradient tangential to the curve. This is done by differentiation of the equation to give 4x-11=dy/dx. dy/dx is the gradient. Now we are given the gradient of the normal. As Mt*Mn=-1 we can find that the tangential gradient is 9. plugging this into the equation we can see that 4x-11=9. rearrage to find x so x=20/4 so x=5