Hiii 
Electric field intensity at A will be E1+E2
and electric field intensity at B will be : E1 – E2
Electric field intensity at A+C will be 
sqrt (E12 + E22 + 2*E1E2 Cos 1200 )
Hope this helps.
						

							
Given, 
q₁ = 10⁻⁸ C 
q₂ = -10⁻⁸ C 
∵ one charge is positive and other charge is negative 
∴ electric field due to q1 and electric field due to q2 are in same direction. 
∴ Electric field at A = electric field due to q₁ + electric field due to q₂ 
= Kq₁/(0.05)² + kq₂/(0.05)² 
= 9 × 10⁹ × 10⁻⁸/(0.05)² + 9 × 10⁹× 10⁻⁸/(0.05)² 
= 2 × 9 × 10/(1/20)²
= 180 × 400 N/C 
= 72000 N/C 
Electric field at B = electric field due to q₁ - electric field due to q₂ 
= kq₁/(0.05)² - kq₂/(3 × 0.05)² 
= 9 × 10⁹ × 10⁻⁸/(1/20)² - 9 × 10⁹ × 10⁻⁸/9 × (1/20)² 
= 36000 - 4000 
= 32000 N/C
Electric field intensity at C = 
Here ,Θ = 120° [ see attachment for understanding ] 
E₁ = kq₁/(0.1)² = 9 × 10⁹ × 10⁻⁸/(0.1)² = 9 × 10³ = 9000 N/C 
E₂ = 9000 N/C [ because magnitude of charges and separation are same ] 
Now, Electric field intensity at C = 
= 9000 N/C