We have three unknowns and only two equations, so it looks as if there is not enough information to solve the problem.
However, the problem does not ask us to solve for a, b, and c. It only asks us to find the value of 2a^2-3b^2+c^2. It turns out the problem is contrived so that it is possible to do that.
One piece of given information relates the values of a and b; the other relates the values of b and c. The common variable there is b. So use those pieces of information to find expressions for a and c in terms of b and substitute them in the expression to be evaluated. It will turn out that all the variable terms cancel out, leaving you with a numerical value for the expression.
