Question

solve the pair of simultaneous equations:y=8-x , 2x^2+xy=-16

Answer 1

this only one equation? it's not a pair

Answer 2

put y=8-x in the second equation:
2x^2+x(8-x)+16=0
then
2x^2-x^2+8x+16=0
x^2+8x+16=0
we see that x^2+8x+16=(x+4)^2 (formula (a+b)^2=a^2+2ab+b^2)
so x is equal to -4
hence y is equal to 12