The ratio of similarity of triangle boy to triangle grl is 1: 2, if bo= x+3 and gr=3x-1, then the length of gr is

The length of GR is 20.

The ratio of BOY/GRL is 1/2.  This gives us part of our proportion we will set up.

BO/GR = (x+3)/(3x-1); now our proportion is

(x+3)/(3x-1) = 1/2

Cross multiply:
2(x+3) = 1(3x-1)

Using the distributive property, we have
2*x + 2*3 = 1*3x - 1*1
2x + 6 = 3x - 1

Gathering all variables on one side, we subtract 2x:
2x + 6 - 2x = 3x - 1 - 2x
6 = x - 1

Adding 1 to both sides, 
6+1 = x-1+1
7 = x

Since x=7, we substitute this into our expression for GR:

GR = 3x-1 = 3(7) - 1 = 21 - 1 = 20