Explanation:
remember, a variable is only a placeholder for an actual value.
so, when we get an actual value, we put it into every location of the variable in the expression and simply calculate.
6.
25/(r - 4) for r = 9
that means
25/(9 - 4) = 25/5 = 5
7.
(5u + 3)/2 for u = 3
that means
(5×3 + 3)/2 = (15 + 3)/2 = 18/2 = 9