The answer is 59.99 because if you divide 180 by 21 you will get 8.57..... Then you need to multiply 8.57..... by 7 to get 59.99
The internal angle sum of a quadrilateral is :
C. 360°
The sum of internal angles of a polygon is determined by a formula :

where n represents number of sides.
In a quadrilateral there are four sides,
So, plugging 4 in the formula :
_____________________________

Written just as an expression it would be:
15-5(p-6)
The answer would be 3z(5x + 7y)