Two small metal spheres with masses 2.0 and 4.0 are tied together by a 5.4--long massless string and are at rest on a frictionless surface. Each is charged to +2.4 .

Answer 1

2.4 what? microcoulombs?

You also need dimensions for the mass and separation. Numbers alone are meaningless.

Is the string tight because of the repulsion force?

What is the question?