Replace the symbols with letters for your own sanity, gather like symbols, and add, multiple and subtract and substitute the other three equations into each other like normal.

So:

r = 4t ....1)

s + 2t = c .....2)

3s = c + t .....3)

-x = 2c-2r .....4)

2) + 2 * 3) =>

7s + 2t = 3c + 2 t, <=> 7s = 3c

into 2) gives

t = (2c/7) .... 5)

so 5) and 1) into 4) -x = 7t - 8t = -t