Triangle abc is a right traingle if side ac=7 and side bc= 10 what is the measure of ab?

For a right triangle with legs legnth a and b and hypotnuse (longes side) legnth c
a²+b²=c²

depends which one is the hyppotonuse
if we assume that bc is the hypotnuse, then
7²+b²=10²
49+b²=100
minus 49
b²=51
sqrt both sides
the missing side is √51

if we assume that the hypotonuse is ab then
7²+10²=c²
49+100=c²
149=c²
sqrt both sides
√149=c

if bc is the hypotonuse, then ab is √51
if ab is the hypotonuse, then ab is √149