The most difficult mathematics is that which you do not know. A surprising amount of mathematics is actually easy once you've learned it. Of course, once you learn the easy stuff, then you have to start tacking the deep stuff, and that gets harder.
An isosceles triangle has two sides of equal length. An isosceles triangle also has two angles of the same measure; namely, the angles opposite to the two sides of the same length; this fact is the content of the isosceles triangle theorem, which was known by Euclid.
An equilateral triangle is an isosceles triangle as it has 2 lengths that are equal and 2 equal sized angles. An isosceles triangle does not necessarily have to have equal lengths and angles do not have to be 60 degrees. So an equilateral triangle is an isosceles triangle but an isosceles triangle may not always be an equilateral triangle.