Given width is 24 less than length.
W=L-24
Perimeter = 2(W+L)=2(L-24+L)=4L-48
But we're also given perimeter = 172'
Therefore 4L-48=172
solve for L
4L=172+48=220
L=55'
W=55-24 = 31'
Check: Perimeter = 2(W+L)=2(31+55)=2*86=172' good!
Answer: The dimensions of the garden are 31' * 55'