F(x)= -4(x+1)^2 + 3 in standard form f(x) = ax^2+bx+c

There are 20 children in the cast of a class play and 8 of the children are boys of the boys 4 have a speaking part in the play and of the girls 8 do not have a speaking part in the play if a child from the cast of the place chosen at random what is the probability that the child has a speaking part

How to solve a simultaneous equation involved with fractions?

Can you me find the missing length. i attached an image.