binary floating point perform substraction and addition 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 ?

You have to define what floating point format you are using. Standard IEEE $64-$bit floating point assigns $53$ bits to the mantissa, giving a precision of about $16$ decimal digits. $10^{38}$ will be represented by some $53$ bit mantissa times a binary exponent. To add to that, you have to match the exponents, and $3.0$ will be shifted far to the right, which means in floating point $10^{38}+3.0=10^{38}$ exactly and when you subtract off $10^{38}$ you will get $0$. As long as the larger number is at least $10^{16}$ times the smaller this will happen.