Two billion people jump up in the air at the same time with an average velocity of 7.0 m/sec. if the mass of
an average person is 60 kilograms and the mass of earth is 5.98 × 1024 kilograms:
a. what is the total momentum of the two billion people?
b. what is the effect of their action on earth?

Well, first of all, remember that the Earth is a sphere (ball-shaped).
So chances are that some of the people will jump in exactly the opposite
direction compared to some of the others, and they'll all cancel each other.

But I understand what you're getting at.  So let's assume that the Earth
is a big cube, and all the jumpers stand on the same face of the cube and
all jump in the same direction.

Momentum = (mass) x (speed) .

The combined momentum of your flash mob is

            (2 billion) x (60kg) x (7 m/s) = 8.4 x 10¹¹ kg-m/s .

In order for linear momentum to be conserved, the Earth must move
in the opposite direction with the same momentum.

 Earth's momentum = ( Earth's mass) x (Earth's speed)

Divide each side by (Earth's mass): 

            Earth's speed = (Earth's momentum) / (Earth's mass) =

                                      (8.4 x 10¹¹ kg-m/s)   /   (5.98 × 10²⁴ kg) =

                                                       1.4 x 10⁻¹³ m/s .

As the crowd jumps upward, transferring their combined momentum
to the Earth, the Earth moves downward, at the speed of

                                  0.000 000 000 14 millimeter per second .