Make each a ratio, and set them equal to each other.
Larger rectangle = 3/5
smaller rectangle = x/3
3/5 = x/3
cross multiply
3/5(5)(3) = x/3(3)(5)
3(3) = x(5)
9 = 5x
Isolate the x, divide 5 from both sides
9/5 = 5x/5
x = 9/5
x = 1.8
1.8 in. is your answer
hope this helps