Question 946011
"....similar...",


We know from that, the corresponding parts are in proportion.  What we do NOT know, if the parts of length 4 and of length 6 are corresponding parts or not.  Are they?  


If those parts do correspond, then your problem question can be answered simply through a proportion.


Check your problem description (which you maybe did not show fully in the help-request) to be sure you know how the parts of your triangles are.  <i>Maybe</i> you may have  {{{4/6=(x+1)/3}}}