Comparing coefficients of , and :
So you've now got three equations in 2 unknowns. Use 2 of them to find values of and . Then plug these values into the third. If it works then, yes, you can do it; if it doesn't, you can't.
So in this case:
Subst into (2):
And test in (1): , which is correct.
So, in this case, yes the first vector can be expressed as a linear combination of the other two. It is:
Use the same method in part (b).