Place each triangle in a fraction (side over side), and set each equal to each other.
6/9 = x/15
Isolate the x, cross multiply
6(15) = 9(x)
Simplify.
90 = 9x
Isolate the x, divide 9 from both sides
90/9 = 9x/9
x = 90/9
x = 10
Find the scale factor, find out what number is being multiplied to make the measurements become from one number to the other. Divide one of the larger triangle's sides with the smaller.
15/9 = 1.67 (rounded).
The scale factor is ~1.67
hope this helps