drawing requires a sign in.
update: Assuming you want voltage with respect to ground.
point a is +5V
Current in loop is CW and is equal to (15–5)v / (4+1)Ω = 10/5 = 2 amps
voltage drop across 1Ω resistor is E = IR = 2 volts
so Vb = –2v
edit: to elaborate a bit on that last number:
start at ground and go CW around the loop. Voltage at point a is –2 volts
next point is –2+15 = +13.
next point is –2+12–8 = +5v
next point is back to ground, 0v.