We know that if two lines are Parallel then both the lines have same Slope.
Given : Equation of 1st Parallel line is -x + 3y = 6
this can be written as :
3y = x + 6
y = x/3 + 2
Comparing with standard form : y = mx + c
we can notice that slope = 1/3
We know that the form of line passing through point (x₀ , y₀) and having slope m is :
y - y₀ = m(x - x₀)
Here the 2nd Parallel line passes through the point (3 , 5)
x₀ = 3 and y₀ = 5 and we found m = 1/3
⇒ y - 5 = 1/3(x - 3)
⇒ 3y - 15 = x - 3
⇒ -x + 3y = 12