Consider the indefinite integral $61+9$x$2$dx$61+9$x$2$dx$.$ Identify the suitable substitution from below and change the variables accordingly and, find the solution. Choose the correct answer from below.$$

Question $4$Answer

a$.$

u$=3$x$611+$u$2$du$6$arctan$($u$)6$arctan$(3$x$)+$cu$=3$x$611+$u$2$du$6$arctan$($u$)6$arctan$(3$x$)+$c

b$.$

u$=3$x$1211+$u$2$du$12$arctan$($u$)12$arctan$(3$x$)+$cu$=3$x$1211+$u$2$du$12$arctan$($u$)12$arctan$(3$x$)+$c

c$.$

u$=3$x$1811+$u$2$du$18$arctan$($u$)18$arctan$(3$x$)+$cu$=3$x$1811+$u$2$du$18$arctan$($u$)18$arctan$(3$x$)+$c

d$.$

u$=3$x$211+$u$2$du$2$arctan$($u$)2$arctan$(3$x$)+$c