Okay, I'll set up a model equation and sow you how it works.
5 + 3 = 8 --- a + b = c
5 + e = f --- a + e = f
So we know c is 8.
Since c = f then f must be 8 also.
Now we just plug in 8 for f on the second equation which would look like
5 + e = 8
We know 5 + 3 is 8
So, therefore our answer is 3
And that's how you solve these equation formats. Hope this helped. Have a great day!