Subtraction two points on elliptic curve.

Question

Suppose Q, T and S are three points on an elliptic curve, such that Q+T = S.
With knowing Q and S, can we compute T?
In other word whether exists subtraction operation on elliptic curve, or not?

Answer 1

Substraction, in any group with additive operation, is just adding the (additive) inverse of an element, i.e.

$$a-b=a+(-b)\;,\;\;\text{with}\;\;-b\;\;\text{being the unique element s.t.}\;\;b+(-b)=0$$

This is always defined in any group (with additive group operation)

Answer 2

Subtraction in an additive group means "inverse". So yes, clearly $T = S - Q = S + Q_{inverse}$.