Unbiased estimator

[rapid iris]

If I have
$$M = -4(X_1 - 2) + 9(X_2 - 1) + aX_3 + b$$
And
$$E(X_1) = E(X_2) = E(X_3) = \mu$$
What values of a and b make this an unbiased estimator

[placid python]

Xotiic

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[rapid iris]

Do I just need E(M) = mu

[sinful urchin]

estimator of what?

[rapid iris]

For what values of a and b is
$$M = -4(X_1 - 2) + 9(X_2 - 1) + aX_3 + b$$
An unbiased estimator for $\mu$

[placid python]

Xotiic

[sinful urchin]

by definition, EM = mu