Answer:
n = 1,724
Explanation:
The interference pattern for two slits for the case of constructive interference is described by the expression
  d sin θ = m λ
Where d is the separation of the slits, lm the wavelength and m an integer
When the slits are submerged the transparent liquid the wavelength of the light changes according to the relationship
  λₙ = λ₀ / n
Where λ₀ is the wavelength in vacuum or air and n is the index of refraction of the material
Let's apply these relationships to our case, write the equations for the two situations
Air
   d sin θ = m λ₀
Liquid
   d sin θ₂ = m λₙ
   d sin θ₂ = m λ₀ / n
Let's pass the variables to the left
   sin θ = m λ₀ / d
  n  sin θ₂ = m λ₀ / d
Let's match the two equations
  sin θ = n syn θ₂
  n = sin  θ /sin θ₂
Let's calculate
  n = sin  35.09 /sin 19.48
  n = 1,724