The area of a rhombus is 84 m

Given:

Area of the rhombus $=84 \mathrm{~m}^{2}$

Perimeter $=40 \mathrm{~m}$

Now, we know: Perimeter of the rhombus $=4 \times$ Side

$\therefore 40=4 \times$ Side

Side $=\frac{40}{4}=10 \mathrm{~m}$

Again, we know: Area of the rhombus $=$ Side $\times$ Altitude

$\Rightarrow 84=10 \times$ Altitude

Altitude $=\frac{84}{10}=8.4 \mathrm{~m}$

Hence, the altitude of the rhombus is $8.4 \mathrm{~m}$.