Resistance and Resistivity ENGAA Question

The notation used is a bit confusing - $A$ here refers to the area of an individual wire (not whole cable), but each of the wires can have a different resistance and density, so they have to be combined correctly to get the right answer.

The expression $6dAL + 3dAL = 9dAL = M$ is simply a statement that the total mass of cable is equal to the sum of the masses of the component wires. When this is rearranged for $A$, it can then be used to substitute for the area of individual Al and Cu wires, remembering that there are 6 Al wires for every Cu wire (hence the factor of $6A$ in denominator of expression for $R_\mathrm{Al}$).