The sum of the real numbers x and y is 15. their difference is 3. what is the value of xy?

X+y=15,
x-y=3
the easiest way is the find out x and y first: 
add up the two equations: 2x=18, x=9
plug x=9 in either equation, you get y=6
so xy=6*9=54
Another way to do it:

(x+y)²=15²=225  =>x²+2xy+y²=225
(x-y)²=3²=9         =>x²-2xy+y²=9
subtract one equation from the other: 4xy=225-9=216, xy=54