Find an equation of the tangent line to the curve at

Question

y = cosx - sinx (pi, -1)

I found y'= -sinx - cosx

and used the equation of the tangent
y = mx +b

I input the values and get -1 - [(-sinx - cosx) * pi] = b

the answer says it is y = x - 1 - pi

I have no idea how the answer got that.

Answer 1

m= -sinPI-cosPI=1
y= x+b
-1=PI+b
b= -PI-1

y=x-pi-1