Question 1098168
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Substitute T = 2 seconds and pi = 3.14 into your basic equation.

You will get

2 = {{{6.28*sqrt(L/10)}}}.


They want you solve it for L.


Square both sides.  You will get


4 = {{{(6.28^2*L)/10}}}     (1)

Then   L = {{{(4*10)/6.28^2}}}.


You make all necessary calculations.
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Solved.