You shouldn't capitalize Kelvin
when you concatenate the output string in your FtoK()
function. It should be as follows:
function FtoK() {
farhen = document.getElementById("C").value
var para1 = document.getElementById("L")
kelvin = ((farhen - 32) / 1.8 + 273).toFixed(1);
para1.innerHTML = farhen + "degress in farhenheit is " + kelvin + " in kelvins" + '<br>';
}