Answer is c) 128
I added the step by step process in the pic attached, I don't know how to explain it very well but you can use binomial theorem for (3x-1)^7
Then you can add up all the coefficients of the calculated values as the question mentions afterwards a^7 + a^6 +..... which means it only wants you to add up the coefficients of the consecutive x terms, otherwise it would say a^7x^7 + a^6x^6 +..... and so on
Hope this helps :)
