ANSWER:
100°
STEP-BY-STEP EXPLANATION:
Since you want to form a 4-sided figure, the sum of all the vertex angles must be 360°.
Therefore, we can calculate the 4th vertex, like this:
[tex]\begin{gathered} \measuredangle1+\measuredangle2+\operatorname{\measuredangle}3+\operatorname{\measuredangle}4=360\degree \\ \\ \operatorname{\measuredangle}4=360\degree-\measuredangle1-\measuredangle2-\measuredangle3 \\ \\ \text{ We replacing:} \\ \\ \operatorname{\measuredangle}4=360\degree-85\degree-70\degree-105\degree \\ \\ \operatorname{\measuredangle}4=100\degree \end{gathered}[/tex]So the measure of the fourth corner is 100°