√21 - √5x-4 = 0
First square both values that are attached to the square root function.
(√21)^2 - (√5x-4)^2 = 0
21 - 5x - 4 = 0
Then combine the like terms 21 and - 4 to get 17.
17 - 5x = 0
Move the - 5x on the left side of the equation to the right side to isolate your variable, x and to remove the (-) negative value attached to your x.
17 = 5x
Then, divide both sides by 5 to make your x value equal to one.
17/5 = 5x/5