27y^3 -343 starts out with 27y^3, and 343 is the cube of 7. Thus, we know immediately that 27y3 -343 is the diffeence of two squares.
The appropriate rule is a^3 - b^3 = (a - b)(a^2 + ab + b^2).
Here, we have 27y3 -343 = (3y - 7)(9y^2 + 12y + 49)
