Newton's 1st law of motion states that a moving object will go in a straight line unless an outside force changes this motion. what outside force prevents the planets from moving in a straight line into space?

Gravity holds the planets in their orbits. 

All objects in space (and everywhere else!) pull each other. This is called gravity. The reason things don't travel in a straight line through space is because everything is pulling each other with gravity.

Yes 21 is the answer. because u use the multiplayer strategy

Like many students at university, you may be unhappy about the results you attain in exams. you may feel that even with all you are doing there must be something more -- or different-- you could be doing to get better grades. we have all at some time or other heard of that student who only studies a couple of hours for final exams and scores a's every time. we stand in awe of those who seem to breeze through without undue effort and seem to need very little in the way of studying to nail an exam. the reasons for success, in what i think are the vast majority of cases, are less esoteric than many students think: successful students consistently apply a series of productive strategies with view to attaining clearly identified learning and grade goals throughout the academic term.