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.