[tex]\qquad \qquad\huge \underline{\boxed{\sf ᴀɴsweʀ}}[/tex]
The factorized form of the given equation is ~
[tex] \boxed{ \sf(3x + 7y) {}^{2} }[/tex]
Let's solve ~
[tex]\qquad \sf \dashrightarrow \: 9 {x}^{2} + 42xy + 49 {y}^{2} [/tex]
[tex]\qquad \sf \dashrightarrow \: (3x) {}^{2} + 2(3x \times 7y) + (7y) {}^{2} [/tex]
Now, as we can see, an Identity is applied here ~
that is ;
[tex]\qquad \sf \dashrightarrow \: {a}^{2} + 2ab + {b }^{2} = (a + b) {}^{2} [/tex]
So, let's use this identity in our next step, taking :
[tex]\qquad \sf \dashrightarrow \: (3x + 7y) {}^{2} [/tex]
