This is a standard type of problem.
The first step is to find the derivative of the function 1 + 2 x - x3 at the point (1,2)
y' = 2 - 3 x2 which evaluates to -1 for x = 1. Thus the slope of the
desired tangent line is -1. The next step is to use the algebra 1 formula for a line when
the slope and one point on it are given:
(y- y1 ) = m (x -x1) for the case here
y-2 = - (x-1) which can be written as
y = -x +3