x - 9 = 15
Add 9 to each side of the equation:
x = 24
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this can not be solved un less with another equation
if n is your number, then 3n - 5 = 16, ie 3n = 21 ie n = 7.
You do the following: 1) Solve one of the equations for one of the variables 2) Substitute this variable in the other equation or equations 3) Simplify This should normally give you one less equation than the original set, with one less variables. For example:
4
As an equation, x = -2y cannot be solved without further information. To solve an equation (or multiple equations), the number of unknowns (x, y etc.) must be equal to or less than the number of equations. In your example you have 2 unknowns and 1 equation, so it does not work.