This 2 x 2 (two equations, two variables) system of equations can be solved by either the elimination or substitution methods. Since it takes one step to write one variable for another, substitution gets the call here.
Call x + y = 12 equation #1
Call 20x + 35y = 315 equation #2
We solve equation #1 for y (you can do it for x too).
x + y = 12
y = 12 - x.
Now we take that solved equation and put it into #2. We solve it for x.
20x + 35 (12 - x) = 315
20x + 420 - 35x = 315
-15x + 420 = 315
-15x = -105
x = 7
Now we take that x = 7 and put it back into an ORIGINAL equation. You can use either one, but equation #1 works well here.
x + y = 12
7 + y = 12
y = 5
Therefore the solution of the system is x = y and y = 5, or (7, 5).