Explanation:
the area of a rectangular is
length × width
we know the area and one side, let's say it's the width (it does not matter, as multiplication are commutative, and a×b = b×a).
so,
7.5 = length × 0.3
length = 7.5 / 0.3 = 25 m
the perimeter of a rectangle is
2×length + 2×width = 2×25 + 2×0.3 = 50 + 0.6 = 50.6 m