I.am.think.of.a.number.if I.add.4 to the number and then mul

I.am.think.of.a.number.if I.add.4 to the number and then multiply the result by 3,the answer is the same as subtracting 5 from the numbers and then multiplying the result by
a) Letting x be the number,write an equation.in terms of
b)Solve the equation and find the number that i am thinking of .(请回答a和b两个问题）