This question may have been asked by a child who has never used algebra.
It is easier to use algebra for a person who knows how BUT here is how another way to do it.
the first one has 2 circles, 2 trangles and another triangle and it adds to 21
the second one has 2 circles, 2 trangles and another circle and it adds to 19
So the triangle must be bigger than the circle by 2. (because that is the only difference)
So if I replace each of the triangles in the second one with a circle plus 2 more I get
5 circles + 2+2 = 19
If 5 circles plus 4 more equals 19 then
5 circles must equal 15
so one circle must be 3 (because 3+3+3+3+3=15)