Are rational numbers closed under addition, subtraction, multiplication, and division?

How do u write a function for the reflection across the y axis

For what values of a is the sum of the fractions 2a−1 / 4 and a−1 /3 positive?

The length of a rectangular plot of land is given by the expression 6x^3y^4. the width of the plot is given by the expression 5x^5y^4z. find an expression to represent the area of the rectangular plot.