P = a + b + c where a < b < c
P = 42 in
b = 18 in
a = x
c = 4x - 1
The equation:
x + 18 + 4x - 1 = 42 |combine items
5x + 17 = 42 |subtract 17 from both sides
5x = 25 |divide both sides by 5
x = 5 (in)
c = 4 · 5 - 1 = 20 - 1 = 19 (in)
Answer: The length of the longest side is 19 in.