Find the values of c so that the intersection of y=-x+6 and y=x/3+c lies
strictly in the first quadrant (not on a coordinate axis).
Solve system of equations to get
x and y in terms of c.
For example:
x=(18-3c)/4 >0 (in Q1)
Solve for c from the inequality:
18-3c > 0
c<6
Similarly, you'd get
y=(3c+6)/4 > 0 (in Q1)
so 3c+6>0
c>2
Combine the two conditions to get
2<c<6
Check all calculations.