Let $A$ denote the set of all $x, y$ values such that $X = 1$. Let $B$ denote the set of all $x, y$ values such that $X = 1, Y \geq 5$. Let $C$ denote the set of all $x, y$ values such that $X = 1, Y < 5$. Then, we have $A = B \cup C$ and $B$ and $C$ are disjoint, i.e., they do not overlap. Therefore,
$$P(A) = P(B) + P(C)$$
This is a general principle you should make sure you remember and understand. Spelling out what $A, B, C$ mean, we have:
$$P(X = 1) = P(X=1, Y \geq 5) + P(X = 1, Y < 5)$$
Independence has nothing to do with this problem, and finding $P(Y \geq 5)$ is not applicable and not possible with the given information. The only pairs of $x, y$ we have any information about are those where $x = 1$, so we can not possibly say anything about $Y$ in general.