us pascal triangle
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
each number is the sum of 2 above it and the 1's are on the outside because only 1 number is above them and its 1
so since its ^4 we use row 4 (actually the 5th because the top row is row zero)
these at the coefficients
so here is
1a^4b^0-4a^3b^1+6a^2b^2-4a^1b^3+1a^0b^4
or easier to see it vertically
notices how exponential start a 4 and 0 until 4 goes down to 0 and 0 goes up to 4
(3x - 1)
a = 3x
b = -1
1a^4b^0 = 1(3x)^4(-1)^0
-4a^3b^1 = -4(3x)^3(-1)^1
+6a^2b^2 ....
-4a^1b^3 .....
+1a^0b^4....
then write it horizontally
I think you see now, when you expand this you will have your answer
