Proof that $M$ is a convex set

Question

I need a help with proving that M is a convex set:

$$M = \{x = (x_1, x_2)^T \mid xa^T ≥ b, x_1 \geqslant 0\}$$

I know the definition of convexity.

I tried to apply this to this set but I don't know how to prove so that it works... Thanks in advance for any tips.