To
understand this picture 2 cones that are stacked one on top of
another. A double-napped cone is created when a generator is rotated
about a fixed vertical axis. A line is created when the cone is cut
by a plane containing the generator. Please see the attachment to
help you understand this.
the expression is 3(15+7). An word problem could be there are 15 apples and 7 bananas. Someone needs to buy 3 times the amount of those. This is represented by that expression.