Let's write out the equation & begin by fully simplifying the right-hand side.
W = 3 ( 2a + b ) - 4
W = 6a + 3b - 4
Now, let's place (a) & it's co-efficient on the left-hand side & the (W) on the right-hand side in order to begin making (a) the subject.
6a = W - 3b + 4
From this, we can simply solve for (a) to find our solution.
a = ( W - 3b + 4 ) / 6
OR
a = ( 1 / 6 ) ( W - 3b + 4 )