The missing side length x is 100 units.
How to find the value of x?
Notice that both triangles have the left vertex in common, so if that vertex has an angle a, then we can write the trigonometric equation:
cos(a) = 36/60
And for the larger triangle, x is the hypotenuse and 60 is the adjacent cathetus to angle a, so we can write:
cos(a) = 60/x
Then we can write the equation:
36/60 = 60/x
Solving for x we get:
x = 60*60/36
x = 100
The missing length is 100 units.