Calculating shear stress in a beam?

shannonxtreme
The equation for shear stress as I know it is VQ/It where V is the shear force, Q is the first moment of area(?), I is moment of inertia and t is the length of the section in question.
My question is, is there a general formula for Q? In my Solid Mechanics notes I've written that Q = ∫y.dA but for some reason this isn't giving me the answers I get from the equation τ= F. (A.ybar/I.t)
Could someone write down the general formula for Q?

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