The sum of those angles must satisfy (n-2)*180 which in this case is a four sided figure so they must sum to 360°
3x-15+2x+2x+2x+24=360 combine like terms on left side
9x+9=360 divide everything by 9
x+1=40 subtract 1 from both sides
x=39°
(No one is stupid, they may be lazy or they may have missed something earlier that now holds them back from grasping what came next, but no one is stupid.)
