Do it in 3 steps:

1: $1000 deposited for 10 years at 7% annually = 13,816.45

2: $13,816.45 @ 7% annually for 10 years (11 to 20) = 27,179.04

So the amount that the new annual deposit of 1000+x must accumulate

to over 10 years (11 to 20) is 50,000 - 27,179.04 = 22,820.96

3: Annual deposit for 10 years (11 to 20) at 7% annually that

accumulates to 22,820.96 = 1,651.72 ; so x = 651.72

This can be easily done by using financial formulas; as example:

(1000 + x)(1.07^10 - 1) / .07 = 22820.96

Solve for x: 651.72

Now YOU do some work: find the formulas