If the system of equations 3x+y=a, 2x+5y=2a, has a solution (x,y) when x=12, compute a.
If x = 12, then your equation simplifies to:
$$36 + y = a$$
$$24 + 5y = 2a$$
We can substitute a for 36 + y:
$$24 + 5y = 72 + 2y$$
$$3y = 48$$
$$y = 16$$
We now know that y is 16, and we substitute that into our first equation:
$$36 + 16 = a$$
$$52 = a$$
$$a = 52$$
So the value of a is 52.
If I am wrong, please tell me as I would be happy to fix it!