Need further explanation thank u

The basic deal is that the isosceles triangle turns out to be an equilateral triangle:

The sum of all angles in any triangle is 180, since 180 minus 60 is 120 and the fact given it to be an isosceles triangle -> has 2 equal sides(length) AND 2 equal angles.
So, the remaining two angles should be equal -> 120/2=60 OR we have one of 60 plus one other (isosceles property) give also 180-120=60.
Either give us the information, this triangle has 3 equal angles and is thus equilateral and has the equilateral property of 3 equal sides -> know 1, know all ;)

Niels

Greetings from Holland

Ps.
You could even solve it with regular Phytagoras split the 60 into two right angled triangles. (30+90+60=180) use soh, cah, toa(*)

(*) I hope the translation is correct, in Holland it's sos, cas, toa.