Solve

x^2 + y^2 = 36
y = 6 - x^2

Can you help me please

You can substitute out the x in the first equation using the second equation. You then have:

6-y+y^2=36.

=> y^2 - y - 30 = 0

Factoring:
(y-6)(y+5)=0
=> y = 6 or -5.

You can then plug in y = 6 or y = -5 into the original first equation to get your values of x.

x^2 + (-5^2) = 36
=> x^2 + 25 = 36
=> x^2 = 11
=> x = sqrt(11).

x^2 + 6^2 = 36
=> x^2 + 36 = 36
=> x^2 = 0
=> x = 0.