I don't understand the question.

The LINEST() function should give you the three coefficients m1, m2, and c in three adjacent cells. (recall that LINEST() is an array function and must be entered accordingly -- see instructions in help file).
Once you have the three coefficients, a simple formula like =[reference to cell containing m1]*[reference to cell containing A]+[reference to cell containing m2]*[reference to cell containing B]+[reference to cell containing c] should be all that is needed to compute V as a function of A and B.