How do you find the radius of a right cylinder with having the height and surface area?
the surface area is 628.3,and the height is 4.2

So we know that surface area is top circle +bottom circle+ (circumfurence times height so) 2(π(radius)^2)+(height(2π(r))) so fill in the blanks
it's kind of hard to see but πr means pi times radius (r)
2πr^2+((4.2)(2)(π)(r))=628.3
factor out the 2 so  the equation would mean
2((πr^2)+(4.2πr))=628.3
divide both sides by 2
πr^2+4.2πr=314.15
what's interesting is that the first few digits of pi are 3.1415 so we aproximate pi as 3.14 and divide both sides by 3.1415
r^2+4.2r=100
subtract 100 from both sides
r^2+4.2r-100=0
factor
(x-8.11812)(x+12.3181)=0
so (if xy=0 then x or/and y must be 0 so)we set them to 0 so
x-8.11812=0
x=8.11812

second one
x+12.3181=0
x=-12.3181
since radii (or any legnth) cannot be negative, we can cross this solution out so

radius=8.11812