Define subtraction for real numbers. How can subtraction be defined for real numbers?
Can it be defined as adding the opposite?
Eg.  A-B = A+(-B)
 A: Generally, (in any ring) subtraction is defined in terms of addition and additive inverses, as in $A-B=A+(-B)$. Since the reals are in particular a ring, you can certainly define subtraction this way. If the approach to the reals you take is axiomatic, then this is pretty much the only possibility. 
However, you may choose not to adopt an axiomatic approach and instead construct the reals by any of (at least seven) different constructions. They will all yield isomorphic models though. If that is done, then it may be possible to transfer a subtraction operation from whatever it is you use to construct the reals. In fact, typically, the addition (and other operations) on the reals is similarly transferred from the underlying structure, and quite often the two ways to define subtraction coincide. 
For instance, if you define the reals as equivalence classes of Cauchy sequence of rationals (suitably defined, to avoid any circular reasoning), then you define addition by $[x_n]+[y_n]=[x_n +y_n]$, and show that it is well-defined, and satisfying the familiar properties. Similarly you define multiplication. You can also similarly define $[x_n]-[y_n]=[x_n -y_n]$, since rational can be subtracted as well. It is then the case that $[x_n]-[y_n]=[x_n]+(-[y_n])$, so the two possibilities coincide. 
