Two numbers have the sum of 1212 and the difference of 518. What are the two numbers?

The answers are 347 and 865. To solve this problem you set up two equations,

x+y=1212 and x-y=518. You then get the second x by itself by adding y to both sides. You then have x+y=1212 and x=y+518. After this you substitute the x into the first equation and have y+518+y=1212. Then simplify to 2y=694 and y=347

Now that y=347 you put it in the original equation. Solve and get that x=865

694

you just subtract 1212 by 518

694+518=1212