jmc

Begin by simplifying the left side: $$3x^2-2x^2-8=42$$ We can combine terms and solve for $x^2$: $$\begin{array}{rl}3x^2-2x^2& =42+8\\ \Rightarrow x^2& =50\end{array}$$ Squaring both sides, we find: $$\begin{array}{rl}(x^2{)}^2& =50^2\\ \Rightarrow x^4& =\boxed{2500}\end{array}$$