Answer
$^{6}$Li: 3 protons and 3 neutrons
$^{7}$Li: 3 protons and 4 neutrons
Work Step by Step
Lithium always has 3 protons, regardless of the isotope. That means that the rest of the atomic mass must be from the neutrons.
For $^{6}$Li, it has an atomic mass of 6.
$6-3=3$ , so there must be 3 neutrons.
For $^{7}$Li it has an atomic mass of 7.
$7-3=4$, so there must be 4 neutrons.
