Write the expression as a monomial in its standard form:

(-3a^7b^2)^4 * 1/27 *ab
an exponent to a power means multiply the exponents
-3^4 a^(4*7) b^*(2*4) * 1/27 *ab
81 a^ 28 b^8 * 1/27 * ab
combine like terms
81/27 a^ 28 *a b^8 b
when multiplying, add the exponents
3 a^ (28+1) b^(8+1)
3 a^29 b^9