HELP ME PLEASE- For the equation b=2a+4, a and b are integers.

The following three assumptions can be made about a and b for the equation above.
Ifa>0,thenb>0.
Ifa=0,thenb≠0.
Ifa<0,thenb<□.
What number is missing from the expression b<□ in the third assumption?

Aspacecraft can attain a stable orbit 300 kilometers above earth if it reaches a velocity of 7.7 kilometers per second. the formula for a rocket's maximum velocity v in kilometers per second is vequalsminus0.0098tplusc ln upper r, where t is the firing time in seconds, c is the velocity of the exhaust in kilometers per second, and r is the ratio of the mass of the rocket filled with fuel to the mass of the rocket without fuel. find the velocity of a spacecraft whose booster rocket has a mass ratio of 20, an exhaust velocity of 2.1 km/s, and a firing time of 15 s. can the spacecraft achieve a stable orbit 300 km above earth?