In rhombus ABCD, AB=18 and AC=26. Find the area of the rhombus to the nearest tenth.
From equation :
$\displaystyle |AB|^2=\left(\frac{|AC|}{2}\right)^2+\left(\frac{| BD|}{2}\right)^2$
you can find value of $\displaystyle |BD|$ an after that calculate area$\displaystyle A$ :
$\displaystyle A=\frac{|AC|\cdot |BD|}{2}$
have you made a sketch?
area, $\displaystyle A = \frac{d_1 \cdot d_2}{2}$
where $\displaystyle d_1$ and $\displaystyle d_2$ are diagonal lengths ... you should also know that the diagonals of a rhombus are perpendicular and bisect each other.
hmmm ... Pythagoras?