The rule would be: Multiply the number of tablespoons by 3
Because based on the graph u see that for 1 tablespoon you have 3 teaspoons and for 2 tablespoons you have 6 teaspoons, for 3 tablespoons you have 9 teaspoons e.t.c.
1 x 3 = 3
2 x 3 = 6
3 x 3 = 9
So it’s just no. of tablespoons x 3 = no. of teaspoons
Hope this helped :)