M(x,y) punctul de intersectie
f(x)=g(x)
7x+21=-5x-3
7x+5x=-3-21
12x=-24
x=-2
f(-2)=7*(-2)+21=-14+21=7
g(-2)=-5*(-2)-3=10-3=7
=>y=7
=>M(-2,7)
