There's actually no such thing as "the formula for a triangle". There are many details of a triangle that you might be asked to calculate. You might be asked to find the perimeter, the area, the angles, or the length of one or more sides. Your question doesn't specify which of these things we're expected to find.
I'm fairly sure they want you to cross multiply then graph. This will give you 'zeroes' which are solutions for x if you graph (4x^3+3x^2)= z it'll give you a zero at -0.75 which means x= -3/4
