To solve this equation, we need to first simplify it by combining like terms.

3a³ - 108 = 7x³ - 28

Next, we can set the two equations equal to each other:

3a³ - 108 = 7x³ - 28

Now we can solve for a and x by isolating each variable on one side of the equation:

3a³ + 80 = 7x³

Subtracting 80 from both sides:

3a³ = 7x³ - 80

Dividing both sides by 3:

a³ = (7x³ - 80) / 3

Taking the cube root of both sides:

a = ∛ (7x³ - 80) / 3

Therefore, the solution to the equation is a = ∛ (7x³ - 80) / 3.