Help. ​

All we need to do here, NSS, is to find the vertex

It appears that it is  ( 50, 150 )  =  (h, k )    

So....the form is

y =  a (x - h)^2 +  k  =  a(x - 50)^2  +  150   

Note  .... "a"  =  - 3/ 50 

So.......Your answer is correct  !!!!.....good job !!!