If x + y - 7 = 0 and 3x + y - 13 = 0, then what is 4x 2 + y 2 + 4xy equal to?

we have, x + y - 7 = 0
⇒ x + y = 7 ...(i)
and 3x + y - 13 = 0
⇒ 3x + y = 13 ..(ii)
By subtracting Eq. (i) from Eq. (ii), we get
3x + y = 13
x + y = 7
----------------------
2x = 6 ...(ii)
∴ x = 3
On putting the value of x in Eq. (i), we get
3 + y = 7
∴ y = 4
Now, 4x² + y² + 4xy
= 4 x (3)² + (4)² + 4 x 3 x 4
= 4 x 9 + 16 + 48
= 36 + 16 + 48 = 100