The problem asks to evaluate the expression $-3a + y$ when $a = 4$ and $y = -5$.

Let's compute this step by step:

$-3a + y = -3(4) + (-5) = -12 - 5 = -17$

So, the value of the expression is $-17$.

Would you like more details on this process or have any questions?

Here are some related questions you can explore:

1. What happens if $a$ and $y$ are both negative?
2. How would the expression change if $a = 0$?
3. What is the result if $y = 0$?
4. Can you rewrite the expression using absolute values?
5. What if $a$ were a fraction?

Tip: Always be careful with signs when plugging in negative numbers!