Explanation:
your teacher wrote most of the solution in this picture.
all we need to do is simplify and transform the equation
28/4 = 21/N
7 = 21/N
7N = 21
N = 21/7 = 3
formally, the ratio of corresponding (side) lengths is the same for all corresponding sides or other lines (like heights) in similar shapes.
so, the rate 28/4 = 7/1 = 7 asked also to 21 and N.
28/7 = 4
and
21/7 = N = 3