Let a=x^2+4. rewrite the following equation i terms of a and set it to equal zero

For this case we have the following expression:
 a = x ^ 2 + 4
 Clearing x we have:
 x ^ 2 = a - 4
 Rewriting we have:
 x = root (a-4)
 
 The expression written in terms of a is given by:
 x = root (a-4)

Figure it out your self