ok so since the triangles are similar, the sides are therefore proportional to each other
you can set this up as the proportion 15/6 = x/8, with x = the missing side
15/6 is equal to 2.5
multiply 8 by 2.5
you get 20
that's the missing side
another way that most people use is the cross-multiplying method
the equation is 15/6 = x/8
15*8 = 120
6 * x = 6x
then you set these two terms equal to each other
6x = 120
then you solve for x and that's your answer
20 is the length of the missing side