Nothing has changed here. Note that this is a circle with radius a (So... wherre will y=a be?). Also, when you differentiate, treat a like the constant that it is.
Find the area of the surface obtained by rotating the circle x^2 + y^2 = a^2 about the line y=a.
The y=a confuses me. What's the answer?
Translate everything down by $\displaystyle a$ units. Therefore rotate $\displaystyle x^2 + (y + a)^2 = a^2$ around the x-axis (I assume you know the formula for doing this).