i need help in work

One of Euler's formulas:  v - e + f  =  2

                                    v - 11 + 7  =  2

                                    v - 4  =  2

                                    v  =  6

i am the writer of this question can i get some help

What does this formula mean?

This is a formula due to Euler:  v = vertices, f = faces, e = edges.

Imagine a cube, for example. It has 8 vertices, 6 faces and 12 edges, so v - e + f is 8 - 12 + 6 = 2

A tetrahedron: v = 4, f = 4, e = 6:   v - e + f:  4 - 6 + 4 = 2

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Thanks Alan.   

I don't remember seeing that formula before.