Floating point substraction

if $ x=1.0e38=1.0 * 10^{38}$ and $ y=3.0$
i want to find $ (x-x)+y $ and $ (x+y)-x$
i think the value of (x-x)+y will be just substract $ x-x=0 + y=3.0 = 3.0$
but how can i perfom addition of different base? $ (x+y)-x$
i think the idea is addition $ (x+y)$ then substract $ -x$ using floating point, i tried to convert $ y=3.0$ to binary such as $ 1.1 * 2^1$
but how about $ 10^{38}$ to binary ?