How can you tell if u are going to get a negative answer when u are adding two integers

What is x and y (use substitution? ) 5x+3y=38 and y=3x+8?

Jacob signs up to work for 2 1/2 hours at the school carnival. if each work shift is 3/4 hour, how many shifts will jacob work? (i just want to double check : |, for anybody that responds! : )

In a graph with several intervals o data how does a constant interval appear? what type of scenario produces a constant interval?