k-2.5 An algebraic expression is just using variables (in this case, k) for unknown numbers. Since the question is asking what the number k minus 2.5 is, your answer is k-2.5.
It is: minus five multiplied by n = -5n
It is 1 - r/7
You cannot write an algebraic expression for that, you need an equation. The equation is n - 6 = 4
It is n + 14.