The slope of the given line = 1 so the slope of the line perpendicular to it will be -(1/1) = -1.
It also passes through point (1, -1) so we use the point slope equation
y - y1 = m(x - x1)
y - -1) = -1(x - 1)
y + 1 = -x + 1
y = - x <--------- that's the answer