The lines y = x/3 + 3 and 5 = x+y intersect at (1.5,3.5)
So feasible region is bounded by the x-axis, the y-axis, the line from (0.3) to (1.5. 3.5), and the line from (1.5,3.5) to ((5,0). You should draw this so you can clearly see the feasible region. Now you need to evaluate = 6x-47 at each corner point of the feasible region.
The corner point that gives you the greatest value of C is the maximum value.
Hope that helps :)