Here's an equation that I don't understand;

1. The formula to find the height of an object 't' seconds after it is thrown upwards is given by h=ut-5t2(squared)

where u is the initial speed with which the object is thrown upwards.

a) Write the equation for an object thrown upwards with an initial velocity of u=80

so, this is h=80t-5t2

b) At what times is h=0? (Put h=0 in the equation and solve for t)

It's the 'b' section that I'm having trouble with....

Thanks