Explanation:
this is a college level question ? elementary school students must be able to solve this.
the perimeter of a rectangle is
2×length + 2×width
as when you walk around a rectangle one full time, you have to walk the long side twice and the short side twice - it has 4 sides, you know.
other words for length : base, ...
other words for width : height, breadth, ...
so, we know
56 = 2×12 + 2×base = 24 + 2×base
now we can make it totally simple right away and divide everything by 2
28 = 12 + base
16 = base
the base (or length) is 16 cm.