The first problem consists in exhibiting a 2-player game, with a Nash equilibrium a*, where a*_1 and a*_2 are the moves in the equilibrium for players 1 and 2. The game must be such that there are moves:
a'_1 \in B_1(a*_2) (thus, a'_1 is a best response for player 1 to a*_2)
a'_2 \in B_2(a*_1) (thus, a'_2 is a best response for player 2 to a*_1)
with both a'_1 different from a*_1, and a'_2 different from a*_2, and such that (a'_1,a'_2) is NOT a Nash equilibrium.
NOTE: for those not familiar with Latex, a_1 is a with subscript 1, and \in is the set membership operator.