Probability PDF function question

Question

If I have this PDF function, and need to solve for the case that $(P≤0.5)$?

I undestand that I need to do integer from 0 to 0.5, but how can I get rid of $a$ in that case?

Answer 1

$$1=\int_{-\infty}^{\infty} f_X(x) \; dx$$ Therefore, $$1=\int_0^{\infty} a\left(2e^{-x}+3e^{-2x}\right) \; dx$$ $$a=\frac{2}{7}$$ Now solve for $P \leq 0.5$.