Hali invested money into an account that is guaranteed to grow$28 each year. she graphs a line that represents the growth of the account. with x representing the number of years since she first opened the account and y representing the number of dollars in it. what is the slope of the line?

If we take the graph with x as number of years and y as amount of dollars in it as mentioned in the equation;
gradient of the graph/ slope (m) = Change in y / change in x 
so if we take a certain portion of the graph 
x is the number of years, since we know the growth for one year,
lets take change in x as 1 year
y is amount of dollars, the change in amount of dollars for one year is 28
therefore change in y is 28
m(slope) = 28(change in y) / 1 (change in x)
              = 28