Explanation:
don't panic. just read carefully and write equations for every relation described in the text.
perimeter = 34 cm = side 1 + side 2 + side 3
side 1 = 2×side 3
side 3 = (side 1) / 2
side 2 = (side 1) + 4
now we use the last 2 equations in the first :
34 = side 1 + ((side 1) + 4) + (side 1)/2
68 = 2×(side 1) + 2×((side 1) + 4) + side 1
68 = 2×(side 1) + 2×(side 1) + 8 + side 1
68 = 5×(side 1) + 8
60 = 5×(side 1)
side 1 = 60/5 = 12 cm
side 2 = (side 1) + 4 = 12 + 4 = 16 cm
side 3 = (side 1)/2 = 12/2 = 6 cm
you see ? it is all just straight forward. no mystery.
the only trick is to find forms of the various equations to have only one variable in the main equation. like we did here with side 1 and side 3.