I hope i helped! ~ Best of Luck!~
Updated!
So the sides ML and MN are exactly the same. Your equation would look something like this...
3x = x + 10
You first subtract x from both sides to isolate the variable.
2x = 10
Divide 2 from both sides.
x = 5
5 is the length of MN.
I hope this helps love! :)
30 plus EF = 180
Ef = 150
circumscribed angle plus the central angle is 180