Probability of a squared transformation

Let $X$ be a random variable with probability density function $f(x) = \begin{cases} \frac38 (x+1)^2 & \text{if } -1 < x < 1 \ 0 & \text{otherwise.} \end{cases}$ If $Y = 1 - X^2$, then $P\left(Y \le \frac34\right)$ equals