If g(x) = -f(x), then the graph of g is obtained from the graph of f by reflecting about the x-axis. Suppose, for example, that f(2) = 3, so the point (2,3) is on the graph of f. Since g(x) = -f(x), then g(2) = -f(2) = -3, and the point (2,-3) is on the graph of g.
since the triangular pyramid has 4 faces and each has an area of 30 
we should multiply that by 4 , so :
