√x + √[ x + 6 ] = 6 subtract √x from both sides

√ [x + 6 ] = 6 - √x square both sides

x + 6 = 36 -12√x + x subtract x from both sides

6 = 36 - 12√x rearrange as

12√x = 36 - 6

12√x = 30 divide both sides by 12

√x = 30/ 12 = 5/2 square both sides again

x = (5/2)^2 = 25 / 4