(x⁻³ - y⁻³)⁻¹³ = [(1/x³)-(1/y³)]⁻¹³ = [(y³-x³)/(x³y³)]⁻¹³ = (x³y³)/(y³-x³) which is not among the forms; instead read the exponent as -1/3: (x⁻³ - y⁻³)⁻¹/³ = (1/x³ - 1/y³)⁻¹/³ = [(y³-x³)/(x³y³)]⁻¹/³ = (x³y³)/(y³-x³) still messy. Re-interpret prompt as (x⁻³y⁻³)⁻¹/³ = (xy)⁻¹.