Question 389662
Hint: To find the mean of a set of numbers, you simply add them up and divide by the number of numbers (in this case 4). 



So this means that you just have to add up 5x, 3/2x, x+9, and -x and divide that sum by 4. Because you're forcing the mean to be -1, the entire expression will equal -1.



So in essence, you just need to solve the equation {{{(5x+(3/2)x+x+9+(-x))/4=-1}}}



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Jim