Find the value of $a$.

Question

please help I'm lost on what numbers to add or what formula to use

Answer 1

Use Pythagoras: $$ a^2 + b^2 = c^2 $$

Pythagorean theorem

$$a^2+12^2=20^2$$

$$a^2 + 144 = 400$$

Subtract 144 from both sides.

$$a^2 = 256$$

you only take the positive answer.

$$a = \sqrt{256}=16$$

Answer 2

I'm answering this post because I think that the animated gif on Wikipedia's article on the Pythagorean Theorem is really cool.

hint

$b=12$ and $c=20$