#(r,theta)# in polar coordinates is #(rcostheta,rsintheta)# in rectangular coordinates.
Hence #(3,(3pi)/4)# is #(3cos((3pi)/4),3sin((3pi)/4))# or #(3*(-1)/sqrt2,3*(-1)/sqrt2)# or #((-3sqrt2)/2,-(3sqrt2)/2)#
and #9.pi)# is #(9cospi,9sinpi)# or #(9xx1,9xx0)# or #(9,0)#
The distance between #(9,0)# and #((-3sqrt2)/2,-(3sqrt2)/2)# is
#sqrt((9-((-3sqrt2)/2))^2+(0-((-3sqrt2)/2))^2)# or
#sqrt((9+(3sqrt2)/2)^2+((3sqrt2)/2)^2)# or
#sqrt(81+27sqrt2+9/2+9/2)# or
#sqrt(90+27sqrt2)# or #3sqrt(10+3sqrt2)# or
= #3sqrt(10+14.142)=3sqrt24.142=3xx4.913=14.739#