Which what ? There are no choices to pick anything from.
If we can massage the equation to get ' y ' all alone on one side and everything else on the other side, then we can read the slope and y-intercept right from the equation. Let's do it.
You said
y - 3 = x + 6
Add 3 to each side:
y = x + 9
And there we have it.
The graph is a straight line. Its slope is 1, and it crosses the y-axis at the point where y=6.
I don’t know if you meant 7 hours or 7 1/2 hours, but if it was in 7 hours, the answer would be around 15 square feet. If it was 7 1/2 hours, it would be 14 square feet.