Assuming y = height of plant
y=0.3x+1.2
7.8 = 1.3x +1.2
or 1.3x = 7.8-1.2
or 1.3x = 6.6
or x = 13/6
or x = 2.166 weeks
but if this question is unrelated to your previous question then,
y should be numbers of weeks, in that case
y = 0.3(7.8) +1.2
y = 3.54 weeks