You can
put this solution on YOUR website!again given the statement
IF a tree has green leaves,,, THEN it produces oxygen
The tree DOES have green leaves ,,, then it does follow that it produces oxygen.
HOWEVER, the initial statement cannot be just an observation, it must define causality.
For instance, the tree has green leaves, it is in my front yard, it provides shade, the birds like it, AND it produces oxygen. No causality is stated therefore, we do not know if any of these conditions lead to it producing oxygen.
Your problem appears to be an observation rather than a conditional statement, therefore we have no proof it is true.