Answer is
x = 0
x = 1
x = 15
Step by step
Given x^4 -16x^3 + 15x^2
First I factored out x^2 from all terms
x^2 (x^2 - 16x +15) = 0
x^2 = 0
√x^2 = √0
x = 0
Now I can factor inside the parentheses
x^2 - 16x +15 = 0
What factor will multiply for product of +15 and also add to sum of -16?
-1 and -15
(x -1) (x - 15) = 0
Solve for each
x - 1 = 0
add 1 to both sides to solve
x -1 +1 = 0 +1
Simplify
x = 1
x - 15 = 0
add 15 to both sides to solve
x -15 + 15 = 0 + 15
Simplify
x = 15