+399 The coefficents of the expansion of a trinomial is a lot like what the binomial theorem states about the coefficients of a binomial.
For example, if you want the coefficient of the a^3*b^2*c term of the expansion of (a+b+c)^6, it is the same as taking 3as, 2bs, and a c, and ordering them which is 6!/(3!*2!*1!).
The thing is we don't know in this problem is how many y^2 or 2ys are multiplied together to create the y^4 term, so we can split this into 3 scenarios:
2 y^2 terms are multiplied together to get y^4
4 2y terms are multiplied together to get y^4
2 2ys and 1 y^2s are multiplied together to get y^4
The coefficent for the first case would be 6!/(2!*4!)*(-5)^4 = 15 * 625 = 9375
The coefficent for the second case would be 6!/(2!*4!)*2^4*(-5)^2 = 15*16*25 = 6000
The coefficent for the third case would be 6!/(2!*3!*1!)*2^2*(-5)^2 = 60*4*(-125) = -30000
Adding all the possible cases together we get: -14625 is the coefficient of the y^4 term
