Also while you're at it,

Side 1 and 2 are 8 cm, side 3 is 15 cm

The perimiter of the triangle is 31 cm, sides #1 & #2 have equal length while side #3 is one cm shorter that twice the length of side one, how long is each side?

Let the length of each equal side  = x

Let the third side  = 2x - 1

And we have

x + x + 2x - 1  = 31    simplify

4x - 1   = 31     add 1 to both sides

4x = 32    divide by 4 on both sides

x = 8 cm   this is the length of each equal side

And the other side  = 2(8) - 1   =  15 cm