You can only subtract like from like, which means that the x^2 terms can be subtracted; the x terms can be subtracted and the constants can be subtracted; but you can't subtract an x term from an x^2 term, for example, you just have to keep them separate in your answer.
3x^2 minus x^2 is 2x^2 (just like 3 marbles less a marble is 2 marbles); 2x minus -3x is 5x. This is like subtracting temperatures: 2 degrees above zero take away 3 degrees below zero. The positive and negative numbers are in different directions and there are 5 degrees between these two temperatures. So 2-(-3)=5. That's why we have 5x. Finally, the constants are just ordinary numbers: -1-4=-5. -1 is negative so subtracting 4 from it makes it more negative. 4 degrees below 1 degree below is 5 below. Or think of the rungs of a ladder. Going down the ladder is the negative direction, while going up is positive. We are one rung from the top (-1) and we go down (subtract) another 4 rungs, so we're 5 rungs from the top.
Now, put all these together: 2x^2+5x-5.