The little box on the angle means it is a 90° angle.
Your starting equation would be 6x +4x +10= 90.
From here, you would combine like terms so the new equation would be 10x +10 =90.
The next step is to subtract 10 from both side so 10x +10-10=90-10 which gets your 10x=80
The last step is to divide 10 from both sides so 10x/10=80/10.
Your final answer would be x=8