a^2 - 5a = -4

Take one half of the red number, then square what you get.

What's one half of - 5?

It's  times -5 or -2.5, right?

That's one of the choices, but that's a wrong choice.  So
is 2.5 a wrong choice.  Those are wrong because you have to 
square -2.5.

What's -2.5 squared?  It's not -6.25 (even though that's listed
as a choice, but it's also a wrong choice).  That's because when 
you square a negative number you get a positive number, not a
negative number.

So what's the only correct choice?

Bet you know now.  :)

Edwin