12.2 no 3

I'm having difficulty understanding how to solve this problem.  Where
should I start?

Well, the first step is to read section 12.2 in the textbook and the notes. The basic notion is that you are trying to write the double integral as an iterated integral.  For part (a), this means that you are integrating first with respect to y and then with respect to x, this means that you need to integrate from the horizontal line at the bottom of the triangle--which gives you the lower limit of integration of the inside integral, to the slanting line at the top of the triangle.  In order to do this, you *must* find the equation of the line relating from the graph of the line; this means that you write a linear equation. for y as a function of x. This function of x becomes your upper limit of integration.
For part (b) you must turn this around--the vertical line on the right gives you the lower limit of the inside integral, and the upper limit becomes the equation for x in terms of y that you get by solving the equation of the slanting line for x.