A matrix is diagonal if

In other words, you want non-zero elements only when the indices of the row and the column are the same: the first element of the first row must be non zero, the second element of the second row must be non-zero, and so on. All other elements must be zero.
Since we are dealing with a radical then x - 5 ≥ 0, for real values
x = 4 is less than 5 , hence is not in the domain
Define the variables: a is the number of apples I can buy g is the number of grapefruits I can buy.
Write the equation: 15 = 0.50a + 2g