y is a function of x if every value of x in the domain maps to exactly one value of y in the range. If you can find a single value of x that maps to two or more values of y, then y is not a function of x.
To figure out if this is possible, suppose that there are two different values of y (which we’ll call y₁ and y₂) that result in the same x. In other words, assume you have y₁ and y₂ such that:
> y₁ ≠ y₂ ; and
> (3y₁ + 5)/(7y₁ - 1) = (3y₂ + 5)/(7y₂ - 1)
Solve this equation, and see if y₁ = y₂. If it does, then this contradicts the assumption that y₁ ≠ y₂, and you can conclude that y *is* a function of x. If you find that y₁ ≠ y₂ holds true, then that shows that y is *not* a function of x.
(3y₁ + 5)/(7y₁ - 1) = (3y₂ + 5)/(7y₂ - 1)
> (3y₁ + 5)(7y₂ - 1) = (3y₂ + 5)(7y₁ - 1)
> 21y₁y₂ - 3y₁ + 35y₂ - 5 = 21y₁y₂ - 3y₂ + 35y₁ - 5
> 21y₁y₂-21y₁y₂ - 5 + 5 = -3y₂ + 3y₁ + 35y₁ - 35y₂
> 0 = -3y₂ + 3y₁ + 35y₁ - 35y₂
> 35y₂ + 3y₂ = 35y₁ + 3y₁
> 38y₂ = 38y₁
> y₂ = y₁
This is a contradiction, so y is a function of x.