After that you have to write integrated answer in square brackets with the limits written at the top bottom and right hand side. You substitute your upper limit into the answer then substitute the lower limit into it. Then you subtract the latter from the former. As you know this is equal to 1, you can equate this to 1 and solve for a
Keep finding the definite integral as you would normally, but using a instead of numerical values. If you equate that to 1, then you should be able to form a quadratic in a.
After that you have to write integrated answer in square brackets with the limits written at the top bottom and right hand side. You substitute your upper limit into the answer then substitute the lower limit into it. Then you subtract the latter from the former. As you know this is equal to 1, you can equate this to 1 and solve for a