Explanation:
the ratio tells us into how many units the total amount is split and then distributed among the different parts or members of the ratio.
3:7:10 means there are 3+7+10 = 20 units of length.
210 ft is the total of all lengths.
this is now split into these 20 units.
that means one unit of length for the ratio is
210/20 = 10.5 ft
the longest side is clearly the one with 10 units in the ratio.
so, it has
10×10.5 = 105 ft