First he split them in to piles of 3, 3, and 2 bricks. Then he weighs both groups of 3 with each other. If they balance he knows the brick is one of the 2 unweighed bricks and he can weigh them to find the heaver one. If the the stacks of 3 bricks do not balance, he will weigh 2 of the 3 bricks. If they balance he will know the brick left unweighed is heavier, or if they do not balance, he will find the heavier one.