What two numbers add up to 9 and multiply to -90?

You solve the equation : t^2 - 9t - 90 = 0 ;
Δ = ( - 9 )^2 - 4 * 1 * ( - 90 ) = 81 + 360 = 441 ;
t1 = ( 9 + ) / 2 = 30 / 2 = 15 ;
t2 = ( 9 - ) / 2 = -12 / 2 = - 6 ;

The numbers are 15 and - 6 ;