Replace all the variables in the expression by their numerical values and calculate the result.
to draw a conclusion from exaiming; to compute an expression:).
That means to look at each statement separately and evaluate after each step.
As it stands (because the rest of the expression may be missing) the evaluation to the constant value of -3 will be the result.
Constant values are expressions as well, still I don't think they are so hard to evaluate... well, in this case it is twelve
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