sin(x) = 5/13

That implies you want to find angle x, not the 5/13, in radians.

5/13 is just a number here. It is the value of the sine of angle x.

If you said "find 5/13 in radians", without showing the sin(x), them 5/13 is already in radians. Or get it in decimals, 5/13 = 0.384615385. So, 5/13 = 0.384615385 radians.

Back to your equation,

sin(x) = 5/13

That is a positive sine value. Sine is positive in the 1st and 2nd quadrants, so, you'd get two answers for x if x is from 0 to 2pi radians only.

sin(x) = 5/13

sin(x) = 0.384615385

Take the arcsines of both sides,

arcsin[sin(x)] = arcsin(0.384615385)

x = arcsin(0.384615385)

Using a calculator,

x = 0.39479112 radians ---in the 1st quadrant.

x = pi -0.39479112 = 2.74680153 radians ----in the 2nd quadrant.