If a regular triangle has a perimeter of 18x + 27 what is an expression for the length of one of its sides

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.