Auniform brick of length 21 m is placed over
the edge of a horizontal surface with a maximum overhang of 10.5 m attained without
tipping. (picture one)
now two identical uniform bricks of length
21 m are stacked over the edge of a horizontal
surface. (picture two)

what maximum overhang is possible for
the two bricks (without tipping)?
answer in units of m.

15.75 m

Explanation:

First, let's look at the top brick by itself.  In order for it not to tip over the bottom brick, its center of gravity must be right at the edge of the bottom brick.  So the edge of the top brick must be 10.5 m from the edge of the bottom brick.

Now let's look at both bricks as a combined mass.  We know the total length of this combined brick is 10.5 m + 21 m = 31.5 m.  And we know that for it to not tip over the edge of the surface, its center of gravity must be at the edge.  So the edge of the combined brick must be 31.5 m / 2 = 15.75 m from the edge of the surface.

the answer is kevlar