As with any "solve for" problem, you undo what has been done to the variable, in reverse order.
Subtract 9, divide by 3, take the 5/3 power
.. 3*(a +4)^(3/5) +9 = 36 . . . . . original equation
.. 3*(a +4)^(3/5) = 27
.. (a +4)^(3/5) = 9
.. a +4 = 9^(5/3)
.. a = 9^(5/3) -4 ≈ 34.9407
