Use the distributive property to expand the parentheses, then isolate #a#:
#3(a+2)+5=2a+4#
#3a+6+5=2a+4#
#3a+11=2a+4#
#3a+11color(blue)-color(blue)11=2a+4color(blue)-color(blue)11#
#3acolor(red)cancelcolor(Black)(color(black)+11color(blue)-color(blue)11)=2a+4color(blue)-color(blue)11#
#3a=2a+4color(blue)-color(blue)11#
#3a=2a-7#
#3acolor(blue)-color(blue)(2a)=2a-7color(blue)-color(blue)(2a)#
#3acolor(blue)-color(blue)(2a)=color(red)cancelcolor(black)(2a)-7color(red)cancelcolor(blue)(color(blue)-2a)#
#3acolor(blue)-color(blue)(2a)=-7#
#a=-7#
That's the solution. Hope this helped!