st-2 ==> 3x+5y+5x+8y = 8x+13y=21
you cannot deduce it to 2x+3y.
can you please explain the highlighted part ??
2nd statement reads - (2) The 6th term of Splus the 7th term equals 21.
And its asking the value of 5th term..which in the sequence i laid out is 2x+3y
now, by the above explanation, we deduced the sum of 6th term and 7th term as 8x+13y = 21.
from this, we cannot derive the value of 2x+3y.. so its not sufficient.
hope it helps.
or example - may think that, if suppose sum of 6th term and 7th term is 4x+6y = 21
so from this you can find the value of 5th term as 2x+3y = 21/2 = 10.5