NOTE: My solution here has +13pi/4 instead of yours as -13pi/4, however, if you just examine this solution, it could help you solve the problem yourself.
13π / 4 needs to be 'reduced' so that we find out where on the Unit Circle you are.
To do this, subtract out 8π / 4 (2π, or a whole Unit Circle) until you can't do it anymore without getting a negative number.
Subtracting out 8π/4 gives you 5π/4.
Sin 5π/4 = -√2 / 2
The reason I chose 8π/4 was because your fraction had 4 in the denominator. If it had, say, 12 in the denominator, the subtraction would have been 24π/12 (still giving 2π, or a whole Unit Circle). Make sense?
