First, we re-order the terms:
2a - 4 1/2a - 2/3b - 1/6b
Now, we combine like terms. 2 - 4 1/2 gives us -2 1/2, and to subtract -2/3 and -1/6, we merely multiply both the numerator and the denominator of -2/3 by 2, and get -4/6. -4/6 - 1/6 is -5/6, so we get:
-2 1/2a -5/6b
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Hope this helps!