Let H = Harry's age now.
Let S = Sally's age now.
Let x = the number of years ago talked about in the problem

The hard part is analyzing this sentence:

Harry is twice as old as Sally was 
at the time when he was as old as Sally is now.

That's quite a mouthfull.  Break that sentence down into two parts:

1.  Harry is twice as old as Sally was...

and 

2.  ...at the time when he was as old as Sally is now.

To part 1 add the words "x years ago"

1.  Harry is twice as old as Sally was x years ago.

Change the words "as old as Sally was x years ago" to "S-x"

1.  Harry is twice S-x.

Now we can write the equation:



To part 2, which is

2.  ...at the time when he was as old as Sally is now.

change the words "at the time when" to "x years ago",
and change "he" to "Harry" 

2. x years ago, Harry was as old as Sally is now.

Change the words "as old as Sally is now" to "S years old"

2. x years ago, Harry was S years old.

So now we can write the equation



The last sentence

Harry's age + Sally's age = 56 years.

is easily changed to

.

So we have this system of three equations and 3 unknowns:



Rewrite the first equation




Rewrite the second equation



Now the system is



Can you solve that system of 3 equations in 3 unknowns?
If not post again asking how.

Answers:  H=32, S=24, x=8.

Checking: 

8 years ago Sally was 16, and Harry is now twice that, since he's 32.
8 years ago Harry was 24, which is how old Sally is now, since she's 24.

That checks.

Edwin