Problem
Square A's left side GH is given as 9 cm, and a square has all sides equal, so A's side length is 9 cm.
Knowing a square has four equal sides is basic shape sense.
4.G.A.1Identify SubproblemsBottoms align, so the 2 cm top gap is how much shorter B's side is than A's: 9 - 2 = 7 cm.
When two squares share a baseline, the height step between their tops is the difference of their sides.
4.MD.A.3Identify SubproblemsSquare B's side is 2 cm shorter than A's 9 cm side, so it measures 7 cm.
A and B stand on the same bottom line, so A's left side splits into a lower part exactly as tall as B's side and an upper part that is the 2 cm gap between the two tops.
That lower part and the 2 cm gap sit end to end along A's side with no space skipped and no overlap, so together they make up A's whole 9 cm side.
Since A's 9 cm side is B's side plus the 2 cm gap, taking that 2 cm back off the 9 leaves B's side by itself.
Subtracting the 2 cm gap undoes the adding of that same gap, so what remains is exactly B's side length.
B's top is 2 cm higher than C's top, so C's side is 2 cm shorter than B's: 7 - 2 = 5 cm.
Same step idea applied to the next neighboring pair.
4.MD.A.3Identify SubproblemsGH and KJ are the far-left and far-right vertical sides, so the gap between them is the whole row: 9 + 7 + 5 = 21 cm.
A picture shows the left and right sides bound the full strip, so the gap equals the summed widths.
4.MD.A.3Draw A DiagramLined-up squares: the step between their tops is the difference of their sides, so you just add the widths!