Rena is older than Ken. The difference of ther ages is 12 and the sum of their ages is 50. Find the age of each

You have two variables:

x: Rena's age

y: Ken's age

And two equations:

x-y = 12

x+y = 50

If you isolate the same variable in each equation, you'll be able to remplace it. I'll isolate x.

x = 12 + y

x = 50 - y

Now, since both equations are equal to x you can also say that they are equal to each other because ...

x=x

12 + y = 50 - y

So you now need to isolate y

2y = 38

y=19

You have Ken's age and to find Rena's, you only need to remplace it in one of your two equations:

x = 12+y

x = 12+19

x = 31