Please Help Me Explain How the Length was found in this equation

Please help me explain how the length was found in this equation. For instance a sentence or two starting out: "The length was found by..."

Thank you so much in advance!

The perimeter of a swimming pool is 96m. The width is 3m more than half the length of the pool. Find the length and breadth of the pool.

96m = l and w (length and width)

p = 96m (p = perimeter)

w = l / 2 + 3m

l= 30m

30 / 2m = 15 + 3m = 18 m

30l + 18w = 96p

(the answer is l = 30 and w = 18)