When x=8 y=20 find y when x=42

Using just the information given, the question can't be answered.

You gave us one point on a graph . . . (8, 20) .

Do you realize how many different lines can go through that point ? ! ?
Each line represents a different function, which would have a different
value when x=42.

You told us nothing about how 'y' depends on 'x' .

If y=20 when x=8, then . . .

Maybe  y =  x + 12 .

Or maybe  y = 2x + 4 .

Or maybe  y = 3x - 4 .

Or maybe  y = 6x - 28 .

From the information you gave us, we have no way to tell.