Please simplify u3.u3.u

I am assuming that 3 here is supposed to be an exponent of u and that the dot is supposed to represent multiplication, so the question should be to simplify
u^3*u^3*u .

Then, use the rules of exponents: when multiplying powers of the same base, you add the exponents together. Then, the result will have the same base, and the resultant exponent will be the sum of all exponents. Also, remember that if there is no exponent next to a variable, then the exponent of 1 (not zero!) is implied:
u^3*u^3*u = u^(3 + 3 + 1) = u^7
In simplified form, the given expression equals
u^7