#### Answer

a. 2a by 3a
b. a by 2a

#### Work Step by Step

The resistance of the block can be calculated by $R=\frac{\rho L}{A}$.
a. To minimize resistance, we want a small length and a large cross-sectional area. We do this by connecting wires to the 2 faces with the largest dimensions, 2a by 3a. This maximizes the area and minimizes the length.
b. Similarly, to maximize resistance, we want a long length and a small cross-sectional area. This is accomplished by connecting the wires to the 2 faces with the smallest dimensions, a by 2a, which minimizes the area and maximizes the length.