Answer :
If you meant:
(2ya)^4=16y^8 raise the left side to the power of four
16y^4*a^4=16y^8 divide both sides by 16y^4
a^4=y^4 take the quartic root of both sides
a=±y
(2ya)^4=16y^8 raise the left side to the power of four
16y^4*a^4=16y^8 divide both sides by 16y^4
a^4=y^4 take the quartic root of both sides
a=±y