graph g(x)=−2x−3.

I dont know how to graph. whats the x and y.

Pick an x, any x. I will pick x = 1

then g(1) = -2(1) - 3 = -5, so we have a point (1,-5)

pick another x, any x, I will pick x = -3
g(-3) = -2(-3) - 3 = 3, so we have another point (-3,3)

plot them, join with a line, extend the line, all done !