After two hours the two plumbers will charge the same amount
Set up your equations
Plumber 1:
y = 25x + 100
Plumber 2:
y = 50x + 50
Set the two equal to each other and solve
25x + 100 = 50x + 50
     -50        -50
25x + 50 = 50x
-25x      -25x
      50 = 25x
   50/25 = 25x/25  <- Divide both sides by 25
       x = 2
After two hours the two plumbers will charge the same amount