#3r^2-5=-10#

#color(white)(a^2a)+5color(white)(aa^2a)+5color(white)(aaa)#Add 5 to both sides

#3r^2=-5#

#(3r^2)/3=(-5)/3color(white)(aaa)#Divide both sides by 3

#r^2=-5/3#

#sqrt(r^2)=sqrt(-5/3)color(white)(aaa)#Square root both sides

#r=+-isqrt(5/3)color(white)(aa)#The negative inside the square root comes out as #i#.

#r=+-isqrt(5/3)*sqrt(3/3)color(white)(aaa)#Rationalize the denominator

#r=+-isqrt(15)/3=+-sqrt15/3i#