430 second spots, 6 15 second spots cost $6050

The multiplication property of zero states that the product of any number and 0 is 0. what must be true about either a or b to make ab=0

Can someone check if i did this correct. it’s number 4 if you’re wondering.

Aprojectile is thrown upward so that its distance above the ground after t seconds is given by the function h(t) = -16t2 + 704t. after how many seconds does the projectile take to reach its maximum height?