I need to know what is the un given side by using Soh Coa Toa

1.68 km

Step-by-step explanation:

You want the horizontal distance (on the ground) from beneath the helicopter to the vertex of the 37 degree angle.  The hypotenuse is given and is 2.1 km.

We want to find the horizontal distance when we already know the angle and the hypotenuse.

That horizontal distance could be called 'x.'   Then:

x/r = adj/hyp = cos 37 degrees, which becomes adj = (2.1 km)cos 37 degrees, which is found using a calculator:  adj = (2.1 km)(0.7987)  =  1.68 km