Réponse :
Explications étape par étape
the aera of the first garden is A=3*10=30 sqm
the aera of the second garden is A'=(3+x)(10+x)
and you know that A'=2A
So you have to resolve (3+x)(10+x)=60
x²+13x-30=0
delta=289 rac. delta=17
x1=(-13+17)/2=2
x2=(-13-17)/2=-15 (value <0 )
The dimensions of the new garden are 3+2=5 m by 10+2=12 m
and the new aera is 5*12=60sqm