For any continuous probability distribution, the total area under the curve indicating the probability density function must be 1.
[tex]\mathbb P(X\le a)=0.75[/tex] is saying that the area to the left of [tex]x=a[/tex] is 0.75. This area is also that of a rectangle.
The height of this rectangle is 0.25, and [tex]\dfrac{0.75}{0.25}=3[/tex], which means the width is 3. Since 4 - 1 = 3, it must be that [tex]a=4[/tex].
