Hello! im new using Julia and i was trying to modeling this:

```
Julia > import Pkg; Pkg.add(“JuMP”)
Julia > using JuMP
Julia > m = Model()
Julia > @variable(m, x1)
Julia > @variable(m, x2)
Julia > @variable(m, x3)
Julia > @variable(m, x4)
julia> @objective(m,Max,-x1 + 7x2 + 14x3 +2x4)
julia> @constraint(m, -x1+x2+x3+x4 >=7)
julia> @constraint(m, x1+27x2+30x3<=89)
julia> @constraint(m,x2-x4==1)
julia> @constraint(m, x3 <= 2)
julia> @constraint(m,x2>=0)
julia> @constraint(m,x3>=0)
julia> @constraint(m,x4>=0)
julia> optimize!(m)
```

when i use “optimize!(m)”, Julia send me this error "ERROR: NoOptimizer()

"

how can i fixed it?

another question: x1 is a free variable, how can i created a free variable?

sorry guys for my bad english, actually im taking english classes to improve it.