You can define the function as follows
f = @(v) (-0.001778*v.^2+0.05333*v).*(v<30);
The (v<30) part simply multiplies the rest by 1 if v is less than 30, and by 0 if v is greater than or equal to 30.
Then construct a vector of values of v running from 0 to 35; call th function with these values to create a vector y, then plot y against v.