Since the original lot is a square and you're given the area remaining and the area for the sari-sari store then you'll have the total area as:
At = 55 + 3(3)
At = 55 + 9
At = 64
Area is defined by the formula
A = s²
where s is the length of the side. Since you already have the area then:
A = s²
64 = s²
s = √64
s = 8m
therefore the original dimension of the lot is 8m by 8m.