The answer for the given above is letter "a, -6r2s4t3". This is obtained by dividing 18 by -3 and subtracting the exponent below with that from above of the same variable. For r, subtract 2 from 4 to get 2. For s, subtract 1 from 5 to get 4 and lastly for t, subtract 3 from 6 to get 3.