It is OK to use letters that have already been defined without redefining them, but any new variable must be defined. In this case the diagram is sufficient an explanation of what the variable is, so it probably does not need to be written out longhand. A parallelogram is a rectangle pushed over and we can figure out its area by looking at the triangles labeled 1 and 2. These two triangles are exactly the same size, so if we were to cut triangle 1 off a rectangle and put it in the position 2 we would have out parallelogram. Probably you will not know the height of your triangle, just the lengths of the sides.
First lets think about the simplest triangle, one with a right angle.
Lets call the sides a, b and c. A Greek called Pythagoras noticed something rather interesting about right triangles like this. Since this gives the area of the square, which seems appropriate. The same is true for the other two sides also. If then, for a given angle these ratios are constant, they must be important, and indeed they are.