x + 0.5y = 3
x^2 - y = 15
from the first equation:-
0.5y = 3 - x
y = 6 - 2x
Substitute for y in second equation:-
x^2 - (6 - 2x) = 15
x^2 + 2x - 21 = 0
x = [-2 +/- sqrt((4 - 4*-21)] / 2
= 3.6904 , -5.6904
Plugging in these into first equation we get
3.6904 + 0.5y = 3 so y = 2(3-3.6904) = -1.3808
-5.6904 + 0.5y = 3 so y = 2(3 + 5.6904) = 17.3808
Solution is x = 3.690, y = -1.381 ; x = -5.690, y = 17.38 ( to 4 s.f's)