Question

a triangle with a base of 6cm and a height of 8cm is similar to a triangle with base 21cm. What is the height of the second triangle

Answer 1

Make a proportion.

Do base over height; $\frac{base}{height}$.

$\frac{6}{8} = \frac{21}{h}$
h is the height of the second triangle. Solve for h.

Cross multiply.
$\frac{6}{8} = \frac{21}{h}$ 
6h = 168
h = 28

The height of the second triangle is 28 cm.