The expression 3/4a - 7/10b represents the subtraction of two fractional quantities where a and b are variables. To find the result of this expression, we must first make sure that the two quantities have a common denominator, which in this case is the least common multiple of 4 and 10. The least common multiple is 40. To convert the two fractions to have a denominator of 40, we multiply both the numerator and denominator of 3/4 by 10/10 and both the numerator and denominator of 7/10 by 4/4, resulting in:
3/4 * 10/10 = 30/40
7/10 * 4/4 = 28/40
The subtraction of the two converted fractions is then:
30/40 - 28/40 = 2/40
So the result of the expression 3/4a - 7/10b is 2/40.