Let the usual speed of the plane to be

\[x\text{ }km/hr\]

The distance to travel

\[=\text{ }1500km\]

since,

Time = Distance/ Speed

As the ques suggests,

\[{{x}^{2}}~+\text{ }250x\text{ }\text{ }750000\text{ }=\text{ }0\]

or,

\[{{x}^{2}}~+\text{ }1000x\text{ }\text{ }750x\text{ }\text{ }750000\text{ }=\text{ }0\]

or,

\[x\left( x\text{ }+\text{ }1000 \right)\text{ }\text{ }750\left( x\text{ }+\text{ }1000 \right)\text{ }=\text{ }0\]

or,

\[\left( x\text{ }+\text{ }1000 \right)\text{ }\left( x\text{ }\text{ }750 \right)\text{ }=\text{ }0\]

Or,

\[So,\text{ }x\text{ }=\text{ }-1000\text{ }or\text{ }750\]

As, speed cannot be negative.

\[x\text{ }=\text{ }750\]

Hence, the usual speed of the plan is

\[750km/hr.\]