we have that
√(87*x)
we know that
87 is equal to------> 3*29
In order for the square root to go further, the number for "x" has to have a factor of 3 or 29
the number 21-------> 3*7
so
√(87*x)------> √(3*29*3*7)------> √(3²*29*7)----> 3√203
the answer is
x=21