Here's the question you clicked on:
steel11
find line tangent to y=xcosx at point (pi, -pi)
just typed y=-1 in. says incorrect
the equation should be y = -x
this is how I calculated it. y = xCosx is the function. taking it's derivative gives the slope to be dy/dx = cosx, which when evaluated at x =pi gives -1. now equation of line is y=mx+c, we know m=-1 and putting y = -pi and x = pi we get c =0. thus the equation of line tangent to the curve is y =-x