Question

Justify each step of the proof.

Answer 1

 I assume that the x is squared 
(on Yahoo!Answers, we often use the caret ^ to show a power or an exponent) 
(think of it as a tiny arrow pointing up, showing that the following number should be seen as if it were raised above the line) 

2(x^2 + 5) = 60 

Can it be x = 5 ? 
we test: 
2(5^2 + 5) = 2(25 + 5) = 2(30) = 60 
it works, therefore x = 5 is a solution. 

Can it be x = -5? 
we test again: 
2[(-5)^2 + 5) = 2(25 + 5) = 2(30) = 60 
it works, therefore x = -5 is also a solution. 

The way you did it, there could be some confusion between step 4 and step 5 (because you do not explain how you go from 4 to 5). 
If you simply "take the square root" on both sides, someone could argue that the "principal" square root of a number is the positive value (+5 only, not -5). On the other hand, if you state: 
(after step 4), we look for all values that give 25 when they are squared, then -5 is allowed. 

The way the conjecture is stated at the beginning, the easiest way is to check both values to find out that, yes, they are both valid solutions.