Your 3√x is the cubic root of x. It can also be represented by x1/3. If we perform this substitution, the equation becomes:
(x5)(1/4)x1/3 = xa
Since (xa)b = xab, we can combine the two first exponents into:
x5/4x1/3 = xa
Since xaxb = xa+b, we can combine the two terms:
x5/4+1/3 = xa
Since our bases are both x, it follows that the exponents must be equal:
5/4 + 1/3 = a
15/12 + 4/12 = a
19/12 = a