Answer
The element is potassium K with the atomic mass 39.0986 amu.
Work Step by Step
The atomic mass of the element is found by multiplying each isotopic mass by its fraction, and then add all of them.
38.964 amu x 0.9326 = 36.3378 amu
39.964 amu x 1.000 * $10^{-4}$ = 0.0040 amu
40.962 amu x 0.0673 = 2.7567 amu
____________________________
Sum: 39.0986 amu
We see in the periodic table of elements that potassium K , has the atomic mass 39.0983 amu, that is close to the number we found (within the experiment error) .
Therefore the element is potassium K with the atomic mass 39.0986 amu.
