6x + 3y = 15 (without plugin)
Now add the plugin for 2
6(2) + 3y = 15 (now solve for y)
12 + 3y = 15 (do 6 x 2)
3y = 3 (subtract 12 from each side)
y = 1 (divide each by 3)
Now the plugin for 5
6(5) + 3y = 15 (now solve for y)
30 + 3y = 15 (do 6 x 5)
3y = - 15 (subtract 30 from each side)
y = -5 (divide each side by 3)
Hope this helped! If it isn't clear just comment your question!