A regular triangle is a triangle with 3 equal sides. A perimeter of a triangle with 3 sides = 3 x length = 3 (length). Since you know that the perimeter is also equal to 18x + 27, therefore,
3 (length) = 18 x + 27
length = (18x + 27) / 3 <-- divided both sides by 3
length = 6x + 9
An expression for the length of one of its sides is 6x + 9.
