Two charges 5x10-⁸C and -3x10-⁸ are …

Case 1 :- Let two charges 5 × 10⁻⁸C and -3 × 10⁻⁸C are placed on a line of points A answer B and C is the point where electric potential becomes zero.
e.g., A---------------C---------------B
Then, use =0
or, 
or, 
Let CA = x then, CB = (16 - x)
so, 9 × 10⁹ × 5 × 10⁻⁸/x + 9 × 10⁹ × (-3 × 10⁻⁸)/(16 - x) = 0
⇒ 5/x = 3/(16 - x)
⇒ 5(16 - x) = 3x
⇒ 80 - 5x = 3x
⇒ 80 = 8x
x = 10
Hence, electric potential is zero at the distance of 10cm from charge of 5 × 10⁻⁸C on line joining the two charges between them.
Case 2 :- Let C is not between the two charges. Then,
A-------------B------C

or, 
or, 
or, 5 × 10⁻⁸/(16 + x) + (-3 × 10⁻⁸)/x = 0
or, 5/(16 + x) = 3/x
or, 5x = 48 + 3x
or, 2x = 48 ⇒x = 24cm
So, electric potential is also equal to zero at distance of 24cm from charge of - 3 × 10⁻⁸C and at a distance of (24 + 16) cm from charge of 5 × 10⁻⁸C,on the side of charge of - 3 × 10⁻⁸C