You're solving for M=0. Your work looks right.
The mass M in grams of undissolved sugar left in a teacup after t seconds is given by: M=10.5-0.4t^2
When will all the sugar dissolve?
M=10.5-0.4t^2
-10.5=-0.4t^2
-10.5/-0.4=t^2
sqroot26.25=t
5.12s=t
can someone plz check if i did it right or can direct me in a right direction. Help would be appreciated.