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