The formula of the area of a trapezoid is:

Area=a[(b1+b2)/2]

Where:

b1= first base

b2= second base

a=height

In the question we are given that:

b1=10

a=7

Area=56

Plugging in these values in the formula to obtain b2:

Area=a[(b1+b2)/2]

56=7[(10+b2)/2]

Expanding:

56=[70+7b2]/2

shifting 2 to the other side:

56*2=70+7b2

112=70+7b2

112-70=7b2

Subtracting:

42=7b2

dividing:

b2=42/7

So:

b2=6

So the length of the second base is 6 cm

Area=a[(b1+b2)/2]

Where:

b1= first base

b2= second base

a=height

In the question we are given that:

b1=10

a=7

Area=56

Plugging in these values in the formula to obtain b2:

Area=a[(b1+b2)/2]

56=7[(10+b2)/2]

Expanding:

56=[70+7b2]/2

shifting 2 to the other side:

56*2=70+7b2

112=70+7b2

112-70=7b2

Subtracting:

42=7b2

dividing:

b2=42/7

So:

b2=6

So the length of the second base is 6 cm