49x^2+42xy+9y^2
To factor a quadratic of the form ax^2+bx+c, you must find two values, j and k, which satisfy two conditions:
jk=ac=441 and j+k=42, so j and k must be 21 and 21.
Now replace bx with jx and kx in the original equation.
49x^2+21xy+21xy+9y^2 now factor 1st and 2nd pair of terms...
7x(7x+3y)+3y(7x+3y) which is equal to
(7x+3y)(7x+3y) which is equal to
(7x+3y)^2