Why not clear the target cell and use two constraints:
C1=1
D1=7
It really is the best way - I have a worksheet to demo this if interested
best wishes
--
Bernard V Liengme
Microsoft Excel MVP
http://people.stfx.ca/bliengme
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"JE McGimpsey" wrote in message
...
One way:

A1: 1
B1: 1
C1: =A1 + B1
D1: =3*A1 + 4*B1


Set Target Cell = D1
Equal to Value of 7
By changing cells: A1:B1
Subject to the Constraints: $C$1 = 1




In article ,
Mike wrote:

How to use Excel to solve for 2 unknown in linear equation, for eg,

x + Y = 1
3x + 4Y =7

Thanks
Mike