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!