The elevation of lake sam rayburn is 164 above mean sea level during the summer if it does not rain the elevation of the lake decreases by 0.5 feet each week

I guess that you want to model the elevation of Lake Sam Rayburn.

During the summer, it is 165 ft above the sea level (the sea level is our position 0ft).

If it does not rain, the elevation of the lake decreases by 0.5ft each week.

So if we assume that there is no rain, we can write the elevation fo the lake as a linear relationship with slope equal to -0.5ft and y-intercept equal to 165ft.

L(w) = 165ft - 0.5ft*w

Where w is the number of weeks without rain, if we have 0 weeks without rain, then the level of the lake remains constant at 165ft above sea level,

L(0) = 165ft - 0.