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.