Rhombus' Side Length

Relevant wiki: Quadrilateral Classification

A rhombus has 4 equal sides, so the two sides given in the question must be equal to each other. So, by , setting up the equation

$3y + 15 = 60 - 6y$

$3y + 6y = 60 - 15$

$9y = 45$

$y = 5$

length of one side is

$3y + 15 = 3 \times 5 + 15 = 30$ .

So the perimeter of this rhombus is 4 times the side length, namely $30\times4=\boxed{120}$ .

A Rhombus is special type of quadrilateral whose all 4 sides are equal
Therefore,
3y+15=60-6y
On solving we get
y=5
Hence length of each side is 30
Hence perimeter = 4×side =120