(-3m²n³)² (-2m³n) ³(1/72m²n³)

Use PP -3m²n³ must be multiplied 2 times and -2m³n must be multiplied 3 times, but 1/72m²n³ does need to (Because there is no exponent, and that is same as that there is 1).

(9m⁴n⁶) (-8m⁹n³)(1/72m²n³)

It can be multiplied with same bases.

-72m¹³n⁹/72m²n³

And it can be divided

-m^13-2n^9-3= -m¹¹n⁶

We can get the results that a is -1, b is 11, and c is 6, so -1+11+6 is equal to 16