If y is directly proportional to x and y = 108 when x=9, what is the value of y when x = 8 and x=7

Y ∝x

y = kx

substitute x and y values

108 = k x 9

because you want to find k on its own you divide both sides by 9

108 : 9 = 12

So k = 9

Now that we have 12 we can use this to find y

y = 12 x 8

You just simply times it because that's what y is

so y = 96 when x=8

To find y when x = 7 you do 7 x 12 which is 84

So y = 84 when x = 7