Find an equation of the tangent plane to the given surface at the specified point. ( z = y cos(x - y) ), ( (9, 9, 9) ).
a) ( z = -y + 9 )
b) ( z = 9x - 9y + 81 )
c) ( z = 9y - 9x + 81 )
d) ( z = 9 cos(9 - 9) )