The first one: 3

The second one: 3+2 * 3 {the two corners below the first triangle lead to a triangle, and a triangle uses three matches}.

The third one: 3+2*3+3*3

..... Look at the bottom. When the number of layers is fixed, the bottom triangle is equal to the number of layers, and the matching number is equal to the matching number above+bottom N*3.

The nth floor: 3+2 * 3+3 * 3 ...+n * 3 = 3 (1+2+3+...+n) = 3 * (1+n) * n/2.

This is the analysis process, and the blank to be filled in the title is the relational expression of the nth layer (replace n with x, that is, Y=3X(X+ 1)/2,

But the first one is not sure what he wants to fill in, and then he can "confirm", but it feels a bit unkind to fill in the blanks like this in math problems. ...