Been trying to solve this for 10 mins

The top and bottom margins of a poster are 6 cm and the side margins are each 6 cm. If the area of printed material on the poster is fixed at 382 square centimeters, find the dimensions of the poster with the smallest area.

my working out so far

xy=382
(x+12)(x+12)

A=xy+12x+12y+144
= 12x+12y+526
=12x +12(382)/x+526
set it equal to zero
12-382(12)/x^2=0

x^2= 382(12)/12
x=√382
x= 19.544820285692065
x= 19.5
Now I find y which is y=382/19.5

Did I make any mistake ???

Question

Been trying to solve this for 10 mins

The top and bottom margins of a poster are 6 cm and the side margins are each 6 cm. If the area of printed material on the poster is fixed at 382 square centimeters, find the dimensions of the poster with the smallest area. 

my working out so far 

xy=382
(x+12)(x+12) 

A=xy+12x+12y+144
  = 12x+12y+526
  =12x +12(382)/x+526
set it equal to zero
12-382(12)/x^2=0

x^2= 382(12)/12
x=√382
x= 19.544820285692065
x= 19.5
Now I find y which is y=382/19.5 

Did I make any mistake ???

anonymous · Answer

how did you go from  
=12x +12(382)/x+526
to

12-382(12)/x^2=0

anonymous · Answer

sorry I think it was supposed to be a + sign

anonymous · Answer

you are also missing the 526

anonymous · Answer

hmm should it be 0? bcos it's a constant?

anonymous · Answer

oh, my bad, you're taking the derivative

anonymous · Answer

no problem...i think i messed up with the algebra stuff...i'm stuck haha

anonymous · Answer

looks right

anonymous · Answer

it makes sense cuz x and y and both around 19.5 so its a square

anonymous · Answer

add 6 to each and you'll get the dimensions

anonymous · Answer

so 25.5 cm x 25.5cm

anonymous · Answer

so that's the answer right?

anonymous · Answer

yep, I believe so

anonymous · Answer

Still got it wrong, but thanks for ur help though! u deserved a medal :)