Since, same width lang ang idadagdag na border, let that be equal to x.
The new dimensions of the garden will be
(7+2x) m by (12+2x) m
Kaya nadagdagan ng 2x, kasi magkabilang side nadagdagan. Up and down, left and right. (see attached)
Incase you need the equation for area,
A = (7+2x) (12+2x ) m²