In other words you don’t separate the √ X + 15  and √ X 
… but put brackets around it and square the whole thing.
√ X + 15  + √ X  = 15
(√ X + 15  + √ X  )² = 15²

I just did it by moving the √ X  other side of the equation before squaring.
 
- note that I can't get a top on the square root thingy so you will have to use your imagination and picture the x+15 under the square root thingy.

I was extremely gifted at GCSE maths and Science.
 
Then I failed A level maths and got D grades in the sciences.
 
So then all I could do was play the piano /failure of an asian.
 
To put in in my school teacher's words 'he wasnt prepared for the difficulty transition between GCSE and A Levels'.

No one told me there would be math.

57Gregy
No one told me there would be math.

I wasn't counting on it either.