Question

a side of the triangle below has been extended to form an exterior angle of 129°. find the value of x.

Explanation:

Step1: Find adjacent interior angle

The exterior angle and its adjacent interior angle are supplementary (sum to $180^\circ$).
$180^\circ - 129^\circ = 51^\circ$

Step2: Calculate $x$ using triangle angle sum

The triangle is right-angled, so the sum of the two non-right angles is $90^\circ$.
$x = 90^\circ - 51^\circ = 39^\circ$

Answer:

$39^\circ$