Roni wants to write an equation to represent a proportional relationship that has a constant of proportionality equal to 7/25. she writes the equation y=x+7/25. what error is roni making?

The error here is that the proportionality constant is added to the other physical quantity (say x) which makes no sense at all. When two quantities are proportional it means that their ratio is always same no matter what. So if y and x are proportional then,
↪ y/x=constant
↪ y/x=7/25
↪ y=7x/25
And this should be the correct relationship between y and x.

The  should be the coefficient of x, not a constant added to x. Like this: