Its tricky!!
Now look
x^2 -y!=2001
i.e. we have too make L.H.S.= R.H.S
So try to make 2001 in the form of x^2 -y!
2001 is +ve vale
i.e. x>y ( x cn be negatv or positve as its square wll alwys be +ve)
Let x= +ve
now x^2>y!
so try too find square whch s greatr than 2001
we got 45....( 45x45= 2025 )
( fore findng squares of multiples of 5 use these formula for ex. 25 x25 = 625
Now right 25 as it is & multiply 2 with 1 highr digit i.e. 3= 2x3=6
so answr s 625
1 more ex. 65x 65 = __25 ( wrte 25 as it is & now multiply 6 with highr digit i.e. 7)
so 65 x 65 = (6x7) 25 =4225 )
now 2025- y!= 2001
i.e. y!= 24
( 24= 1x2x3x4 = 4! )
therefore
x= 45
y=4
