Just plug in the values into the given equation to solve:
V = a + bx
a = 17.8 V and b = -3.40 V/m
For part A:
first part, at x = 0 you would have
V = 17.8 V + -3.40(0) = 17.8 V
similarly the next part plug in x = 3m.
V = 17.8 V + -3.40(3) = 7.6 V
and then x = 6m
V = 17.8 V + -3.40(6) = -2.6 V
For part B:
the formula for electric field is:
E = - dV / dx (which means its the negative of the slope of the equation for the potential difference.
V = 17.8 V + -3.40x
So the electric field would just have magnitude of 3.4 V/m, which is the slope.
This number would be the same no matter what x it is at. and the direction is positive for all of them.