We first have to make an equation that represents this equality. To do it we simply have to add the values that give us with x and this will result in 180 °
15° + 73° + x = 180°
once we have the equation what we have to do is solve x, so we will pass all the terms that are adding to x by subtracting 180
x = 180° - 73° - 15°
now that we have cleared the x we only have to perform the indicated operation and we will obtain the value of x
