[tex]\mathbb E(X)=\int_7^{26}\dfracx{26-7}\,\mathrm dx=\dfrac{33}2=16.5[/tex]
which follows from the definition for expected value,
[tex]\mathbb E(X)=\int_{-\infty}^\infty xf_X(x)\,\mathrm dx[/tex]
where [tex]f_X(x)[/tex] is the probability density function for [tex]X[/tex].
