LOL, I should copy and past :)
The sum of angles for any triangle is always 180°, so in this case we can say:
x+3x+x+15=180 combine like terms on left side
5x+15=180 subtract 15 from both sides
5x=165 divide both sides by 5
x=33°
That of course is just x, the angles show specifically can be found by using that value of x for the others...
x, 3x, x+15 are 33°, 99°, 48° respectively