Help for maths SUM

[aliatifkhan777]

I know it's dumb to ask this, but i forgot how to expand -__-

please explain me this

(3x+y) Squared

 

[anahita16]

(3x)^2 + 2(3x) + y^2 = 9x^2 + 6xy + y^2

 

[Silent Hunter]

a^2 + 2ab + b^2 = (a + b)^2

use the above formula