10
It is said that the diagonals of a rectangle are congruent or equal.
So if AC and BD are the diagonals of the given rectangle, then they are congruent.
[tex]\overline{AC} \cong \overline{BD}[/tex]
[tex]4x-30=x[/tex]
[tex]4x - x = 30[/tex]
[tex]3x = 30[/tex]
[tex]\frac{3x}{3} =\frac{30}{3}[/tex]
[tex]x = 10[/tex]
So if the measure of diagonal AC = 4x - 30, then:
[tex]4(10)-30\\40-30 = \underline{\bold{10}}[/tex]
[tex]\overline{AC} =\overline{BD}=10[/tex]
