[list=1][*]Plot the planes with equations:[br][b]x+y+z=180[br]y=4x-50[br][/b]z=x-40[br][br][/*][*]Select [b]Tools[/b] in the left-hand panel and use the [b]Intersect two surfaces[/b] tool to find the line of intersection of two of the planes.[br][br][/*][*]Use the [b]Intersect two surfaces[/b] tool again to find the line of intersection a different pair of planes.[br][br][/*][*]Select [b]More [/b]and use the [b]Intersect[/b] tool to find the point of intersection of these lines. [br][br][/*][*]Select [b]Algebra[/b] in the left-hand panel to view the point of intersection.[br][/*][/list]