Anyone can help me please

Which shows the correct solution of the equation 1/2a+2/3b=50, when b=30?

Find the domain of the following expression: 1/(√x)-2

Jada has a meal in a restaurant she adds up the prices listed on the menu for everything they ordered and gets a subtotal of $42.00. when the check comes, it says they also need to pay $3.99 in sales tax. what percentage of the subtotal is the sales tax