Start by representing the lengths of the three sides of your triangle:
first side: x-4 (inches) second side: x (inches) third side: (x-4) + 3 (inches)
Add these three quantities up to obtain a formula for the perimeter, and set your sum equal to the given perimeter (15 inches):
x-4 + x + x-4 -3 = 15
3x-11=15 3x=26 x=26/3
Thus, the length of the 2nd side is 26/3; that of the first side is 26/3-4, or 14/3; and that of the third side is 14/3+3, or 23/3 (all measurements in inches).