Explanation:
so, the whole thing starts out as
x/4 = 10
the next 2 steps are written by your teacher :
multiply both sides now by 4
4 × x/4 = 10 × 4
on the left side (as your teacher stroke through)
4 / 4 = 1
leaving x all by itself.
on the right side
10 × 4 = 40
and you get
x = 40
and there is your result.