Question 152453
The plant's height is given by
{{{h = 8 + 0.8w}}}  where h is hte height and w is the number of weeks

You are asked to say when plant will be taller than 13.6cm

{{{13.6 < 8 + 0.8w}}}
{{{5.6 < 0.8w}}}
{{{5.6 / 0.8 < w }}}
{{{7 < w}}}
So anytime after 7 weeks, gets the job done!