If all the sides are the same then we know that all the angles must be equal as well. All the angles must equal a total of 180 in a triangle, so 180 / 3 = 60, now we know all the angles equal 60. So knowing that we can set the equation equal to 60.
(5x - 40) = 60 simple algebra from here on
+40 +40
5x = 100
/5 /5
x = 20
and now that I'm looking at your answer choices I'm confused lol
that 5x on the outside should be equal to 180 - 60 which is 120
120 / 5 = 24
so x can't be both 20 and 24, my best guess would be 22, sorry m8