Answer:
LCM(a, b) means the least common multiple between a and b.
LCM(a,b) = a*b/GCF(a,b)
Where GCF(a,b) = greatest common factor between a and b.
Then the smallest common multiple between two prime numbers is the product between those prime numbers (Because the only common factor between prime numbers is 1).
LCM(a,b) = a*b = 783.
Now, to find the values of a and b, we can divide 783 by different primes, and see when the quotient is an integer:
(I ignore the number 2 because 783 is not even, so i will start with the prime 3)
783/3 = 261
Then the two primes are 3 and 261
We also know that a > b
Then:
a = 261
b = 3.
2*a*b - 3*a = 2*261*3 - 3*261 = 261*3*(2 - 1) = 261*3 = 783