From the Pythagorean Theorem, we know that a^2+b^2=c^2 28^2+21^2=(5x)^2 784+441=25x^2 1225=25x^2 49=x^2 x=7 The length of the hypotenuse would be 35m.

[latex]a=21 \ m , \ b=28 \ m , \ c=5x \ m \\ \\c^2 =a^2 + b^2\\ \\(5x)^2 =21^2 + 28^2\\ \\25x^2=441+784\\ \\25x^2=1225/:25\\ \\x^2=49\\ \\x=\sqrt{49}\\ \\x=7 \ m[/latex]