Voltage potential difference

A negative point charge q1 = -4.0 nC is at the origin and a point charge q2 = +6.0 nC is on the
+x-axis at +0.40m. Point A is at -0.20m and point B is at -0.50m on the x axis. What is the potential difference Va-Vb? Which point is at a higher potential, A or B?

Note: for the first part of the problem I had to find the Electric field for point A and B.
Ea=749 N/C +x
Eb=77.3 N/C +x

I'm not sure what equation to use to find the potential difference.

Work out the potential from each charge at each point by simple addition of potentials. Then subtract potential at B from the potential A

Potential at A from q1 = kq/r = 9x10^9 x (-4.0x10^-9) / 0.2 = -180V
Potential at A from q2 = kq/r = 9x10^9 x (+6.0x10^-9) / 0.6 = +90V
Total potential at A, Va = -180+90 = -90V

Potential at B from q1 = kq/r = 9x10^9 x (-4.0x10^-9) / 0.5 = -72V
Potential at B from q2 = kq/r = 9x10^9 x (+6.0x10^-9) / 0.9 = 60V
Total potential at B, Vb = -72+60 = -12V

Va - Vb = -90 - (-12) = -78V

B is at the higher potential because -12V is 'less negative' than -90V ('less negative is equivalent to being 'more positive')