we can set up equations for this one. let's define a variable x, to be the first piece of pipe cut.
so, we know the sum of all the pieces is 84.
x {this is for the first pipe} + x - 3 {this is for the second pipe} + x + 4 {this is for the third pipe} = 84.
now, we can combine the like terms, and then isolate the variables.
3x + 1 = 84
3x = 84 - 1
3x = 83
x = 83/3
well, the numbers did not end up quite as nice, but we can still solve for each piece.
the first piece is just 83/3.
the second piece is 83/3 - 9/3 = 74/3.
the third piece is 83/3 + 12/3 = 95/3.
if you would like to double check, you can add up all of those and see if they add up to 84!
hope this helped! please let me know if you are still confused