loop and if statement

let us suppose
there are three equation and three variable
and want to solve for several year
and Equation1=G=10
Equation2=G=15
Equation3=G=16

whether it is possible or not if Equation 2=G=-15 with negative constraints in second year but I want to drop these equation2 because i need only positive constraints in second year and solve for only 2 variable by putting if condition in loop