What formula would i use to find a?

For this case by cosine theorem, we can use the following formula to find the value of a:
 a ^ 2 = (6.3) ^ 2 + (15.10) ^ 2 - 2 * (6.3) * (15.10) cos (70)
 Clearing a we have:
 a = root ((6.3) ^ 2 + (15.10) ^ 2 - 2 * (6.3) * (15.10) cos (70))
 
 A formula to find A is:
 a = root ((6.3) ^ 2 + (15.10) ^ 2 - 2 * (6.3) * (15.10) cos (70))