If we double the third equation we get 2x + 4y - 8z = -66. So we can eliminate x and y between the first and this new equation, simply subtract one from the other. Easier to subtract from the first equation to give us 11z = 55, so z = 5. Now substitute this value into equations 1 and 2. 2x + 4y = -26 and 4x - 3y = 14. Double the first of these and we get 4x + 8y = -52. Subtract the other equation from it and we get 11y = -66, so y = -6. Take any equation and substitute for y and z. Let's take the last equation x - 12 - 20 = -33, from which x = -1. Try the values out in each original equation to make sure they're right. In problems of this sort try to make the work easy by spotting coincidences, like the fact that the first and third equations eliminate two variables at once because of a simple multiple, 2 in this case.