Here is my sage advice to you, Silvia95. Do not redraw structures!!! Use the method described here. Transcribing structures to place the lowest priority group in the rear will invariably lead to errors. Leave the structure where it is and use your hands as tools to determine chirality. Make a "thumbs up" gesture with either hand and point the thumb from the asymmetric carbon to the lowest priority group. Your fingers should point in descending order from highest priority to the next to lowest priority. If not, you are using the wrong hand. Right hand, R; left hand, S.
C2 of dibromide a has the hydrogen in the rear. The fingers of your left hand show Br>C3>C1. C2 in dibromide b has the hydrogen in the front. Now the thumb of your left hand points toward you and Br>C3>C1. Using the same technique for C3 of a and b, your right hand fingers reveals Br>C2>C4. These two structures are identical, (2S,3R)-2,3-dibromopentane. If you rotate either structure by 180o about a vertical axis in the plane of the screen, the two structures will superimpose.
