The value of y varies directly with x, and y=28 when x=7. find y when x=4 by first finding k.

Direct proportionality is the type of proportionality where an increase in one quantity or variable causes a corresponding increase in another quantity. 
Therefore; y varies directly as x
Hence; y α x
             y =k x where k is a constant 
if y= 28 and x = 7 , then k = 28/4 =4
hence, y = 4 k
when x = 4, then y= 16