The fractions have unlike denominators. First, find the Least Common Denominator and rewrite the fractions with the common denominator.
LCD(3/4, 2/5) = 20
Multiply both the numerator and denominator of each fraction by the number that makes its denominator equal the LCD. This is basically multiplying each fraction by 1.
(3/4×5/5)−(2/5×4/4)=?
Complete the multiplication and the equation becomes
15/20−8/20=?
The two fractions now have like denominators so you can subtract the numerators.